Bring back the "|" symbol and eliminate search rakes

This commit brings back the "|" symbol for pipeline separator
while retaining the "|>" as an option.  The latter is needed to
exit a SQL expression back into a pipe when a SQL expression
is immediately followed by a shortcut that begins with an identifier
(implied-yield expression or agg function).

For any valid query without shortcuts, there is never an ambiguity
between the bitwise OR "|" and pipe separator because keywords cannot
be used as identifiers.  The argument that such ambiguity is
present made in the Google SQL pipes paper presumably assumes a
one-token lookahead parser, which is not the case here.

Toward this end, we changed our grammar to disallow keywords as
identifier and added the backtick-string syntax to escape any
string as identifier.

Furthermore, we arranged for the expression grammar to include
"|" for bitwise OR in SQL expression but omit it in any
non-SQL pipe operator.  This means any valid SQL queries will
continue to be valid SuperSQL queries (inclusive of bitwise OR).
Moreover, expressions in pipeline operators do not have any
pipe-character ambiguity with shortcuts because bitwise-OR is omitted.
We can add bitwise functionality as named functions in place of the
archaic C-style syntax (which can work in both SQL and pipeline
operators).  Best practice will be to use these functions over
the old bitwise syntax.

We also removed search syntax as a shortcut and now require a "?"
(or "search" keyword) to signal the keyword search operator.
This means typos that happen to compile into unintended searches
are no longer accepted by the grammar thereby eliminating the so-called
search rake.

cmd/super/ztests/ast.yaml

@@ -1,12 +1,12 @@
 script: |
-  super -C -c a
+  super -C -c "?a"
   super -C -I query.zed
   super -C -I query.zed -c a
 
 inputs:
   - name: query.zed
     data: |
-      q
+      ? q
 
 outputs:
   - name: stdout

cmd/super/ztests/diropt1.yaml

@@ -1,4 +1,4 @@
-script: super -f zeek -split out -c "*" -
+script: super -f zeek -split out -
 
 inputs:
   - name: stdin

cmd/super/ztests/diropt2.yaml

@@ -1,5 +1,5 @@
 script: |
-  super -z -split out -o foo -c "*" -
+  super -z -split out -o foo -
 
 inputs:
   - name: stdin

cmd/super/ztests/stop-on-error-1.yaml

@@ -1,5 +1,5 @@
 script: |
-  ! super -c "*" good.jsup bad.jsup
+  ! super good.jsup bad.jsup
 
 inputs:
   - name: good.jsup

cmd/super/ztests/stop-on-error-2.yaml

@@ -1,5 +1,5 @@
 script: |
-  super -z -e=false -c "*" good.jsup bad.jsup
+  super -z -e=false good.jsup bad.jsup
 
 inputs:
   - name: good.jsup

cmd/super/ztests/stop-on-error-3.yaml

@@ -1,6 +1,6 @@
 # Second input has bad middle line (detection succeeds).
 script: |
-  ! super -z -e=false -c "*" good.jsup bad.jsup
+  ! super -z -e=false good.jsup bad.jsup
 
 inputs:
   - name: good.jsup