Make choice between newtype and type configurable

[edsko]

We can do this at two levels:

• A global default: do we want to use Haskell newtypes or type aliases for C typedefs (which would then apply to typedef int my_clock_t as well as typedef struct foo foo_s equally). Currently the default here is to use newtypes everywhere (see also Is typedef a type or a newtype or? #173)
• More targeted overrides of the defaults, when using hs-bindgen as a library (see below).

Background:

In #306 we discussed what to do with things like

struct foo { int x; int y; };
typedef struct foo foo_s;

The conclusion was that we'd want to generate a Haskell newtype for foo_s, unless foo_s and foo are spelled identically.
We could alternatively make foo_s a Haskell type alias for foo, but this feels somewhat inconsistent with #173; in particular, although it is hard to imagine that we might want different type class instances for foo and foo_s in this example, it's entirely conceivable we might have something like

typedef foo_s some_special_use_of_foo

elsewhere in the header file for which we do want to generate a newtype. One could imagine heuristics such as

• if the name foo and foo_s are "sufficiently similar" (_s suffix, ..?)
• if the foo and foo_s are "suffiently closely" defined (same C definition, consecutive lines, ..)

but unless we can find rules that seem to apply quite universally, it might be better to leave these to individual users (using hs-bindgen`-as-a-library); we should however provide the necessary hooks to implement such rules.

[phadej]

I realise that we need this for this example:

// in stdbool.h
#define bool _Bool

if we would treat #defines similarly as typedefs (currently we generate newtype for both), we'd generate (using current name mangling rules)

newtype CBool' = MkCBool' CBool

but in this particular case we really want type, or maybe not generate any Haskell representation (i.e. inline alias, a third option between newtype and type).

For now we can hardcode bool case, but I think the same machinery can be used for it as for "user" defined types.