Make fbuffer_inc_capa easier to inline

With the extra logic added for stack allocation, and especially the
memcpy, it became harder for compilers to inline.

This doesn't fully reclaim the speed lost with the stack allocation,
but it's getting closer.

Before:

```
== Encoding twitter.json (466906 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json   160.000 i/100ms
                  oj   225.000 i/100ms
Calculating -------------------------------------
                json      1.577k (± 2.0%) i/s  (634.20 μs/i) -      8.000k in   5.075561s
                  oj      2.264k (± 2.3%) i/s  (441.79 μs/i) -     11.475k in   5.072205s

Comparison:
                json:     1576.8 i/s
                  oj:     2263.5 i/s - 1.44x  faster

== Encoding citm_catalog.json (500298 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json   101.000 i/100ms
                  oj   123.000 i/100ms
Calculating -------------------------------------
                json      1.033k (± 2.6%) i/s  (968.06 μs/i) -      5.252k in   5.087617s
                  oj      1.257k (± 2.2%) i/s  (795.54 μs/i) -      6.396k in   5.090830s

Comparison:
                json:     1033.0 i/s
                  oj:     1257.0 i/s - 1.22x  faster
```

After:

```
== Encoding twitter.json (466906 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) [arm64-darwin23]
Warming up --------------------------------------
                json   213.000 i/100ms
                  oj   230.000 i/100ms
Calculating -------------------------------------
                json      2.064k (± 3.6%) i/s  (484.44 μs/i) -     10.437k in   5.063685s
                  oj      2.246k (± 0.7%) i/s  (445.19 μs/i) -     11.270k in   5.017541s

Comparison:
                json:     2064.2 i/s
                  oj:     2246.2 i/s - 1.09x  faster

== Encoding citm_catalog.json (500298 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) [arm64-darwin23]
Warming up --------------------------------------
                json   133.000 i/100ms
                  oj   132.000 i/100ms
Calculating -------------------------------------
                json      1.327k (± 1.7%) i/s  (753.69 μs/i) -      6.650k in   5.013565s
                  oj      1.305k (± 2.2%) i/s  (766.40 μs/i) -      6.600k in   5.061089s

Comparison:
                json:     1326.8 i/s
                  oj:     1304.8 i/s - same-ish: difference falls within error
```

ext/json/ext/fbuffer/fbuffer.h

@@ -33,7 +33,7 @@ static void fbuffer_append(FBuffer *fb, const char *newstr, unsigned long len);
 #ifdef JSON_GENERATOR
 static void fbuffer_append_long(FBuffer *fb, long number);
 #endif
-static void fbuffer_append_char(FBuffer *fb, char newchr);
+static inline void fbuffer_append_char(FBuffer *fb, char newchr);
 #ifdef JSON_GENERATOR
 static VALUE fbuffer_to_s(FBuffer *fb);
 #endif
@@ -66,29 +66,34 @@ static void fbuffer_clear(FBuffer *fb)
 }
 #endif
 
-static inline void fbuffer_inc_capa(FBuffer *fb, unsigned long requested)
+static void fbuffer_do_inc_capa(FBuffer *fb, unsigned long requested)
 {
-    if (RB_UNLIKELY(requested > fb->capa - fb->len)) {
-        unsigned long required;
+    unsigned long required;
 
-        if (RB_UNLIKELY(!fb->ptr)) {
-            fb->ptr = ALLOC_N(char, fb->initial_length);
-            fb->capa = fb->initial_length;
-        }
+    if (RB_UNLIKELY(!fb->ptr)) {
+        fb->ptr = ALLOC_N(char, fb->initial_length);
+        fb->capa = fb->initial_length;
+    }
+
+    for (required = fb->capa; requested > required - fb->len; required <<= 1);
 
-        for (required = fb->capa; requested > required - fb->len; required <<= 1);
-
-        if (required > fb->capa) {
-            if (fb->type == STACK) {
-                const char *old_buffer = fb->ptr;
-                fb->ptr = ALLOC_N(char, required);
-                fb->type = HEAP;
-                MEMCPY(fb->ptr, old_buffer, char, fb->len);
-            } else {
-                REALLOC_N(fb->ptr, char, required);
-            }
-            fb->capa = required;
+    if (required > fb->capa) {
+        if (fb->type == STACK) {
+            const char *old_buffer = fb->ptr;
+            fb->ptr = ALLOC_N(char, required);
+            fb->type = HEAP;
+            MEMCPY(fb->ptr, old_buffer, char, fb->len);
+        } else {
+            REALLOC_N(fb->ptr, char, required);
         }
+        fb->capa = required;
+    }
+}
+
+static inline void fbuffer_inc_capa(FBuffer *fb, unsigned long requested)
+{
+    if (RB_UNLIKELY(requested > fb->capa - fb->len)) {
+        fbuffer_do_inc_capa(fb, requested);
     }
 }
 
@@ -113,7 +118,7 @@ static void fbuffer_append_str(FBuffer *fb, VALUE str)
 }
 #endif
 
-static void fbuffer_append_char(FBuffer *fb, char newchr)
+static inline void fbuffer_append_char(FBuffer *fb, char newchr)
 {
     fbuffer_inc_capa(fb, 1);
     *(fb->ptr + fb->len) = newchr;