Add LibC (WIP)

This commit includes the "easy" types.  Once merged, we can start work
on integrating using of the library while simultaneously filling in the
details.

Notes:

* The `x86_64` architecture `Arch` module is put in place, though no
  architecture-specific types that require it are implemented yet.  TODO
  comments marked "`arch`" need to be implemented in `Arch` modules.
* As decided on in our last call, integral types with a specific bit
  width are implemented using corresponding Haskell types.  This allows
  them to be implemented in general, not in `Arch` modules.
* Documentation indicates that `stdint.h` should define types
  `int_fast8_t`, `int_fast16_t`, `int_fast32_t`, and `int_fast64_t` (as
  well as the unsigned versions), but Musl does *not* define
  `int_fast16_t` or `int_fast32_t` (or the unsigned versions).  I went
  ahead and implemented them in the module anyway.
* No types from `stdatomic.h`, `stdbit.h`, or `stdckdint.h` are
  implemented yet.  These are not (yet) supported by Musl.
* The types implemented use names that match those in `base`.  I can
  rename them to use our default name mangling rules if we decide to do
  that instead.  I did not implement any structures or enumerations yet;
  I will do so once we decide on naming.

Author: TravisCardwell

hs-bindgen-patterns/hs-bindgen-patterns.cabal

@@ -34,15 +34,20 @@ library
   import:
       lang
   exposed-modules:
-      HsBindgen.Patterns
       HsBindgen.ConstantArray
+      HsBindgen.Patterns
+      HsBindgen.Patterns.LibC
   other-modules:
       HsBindgen.Patterns.Arithmetic
       HsBindgen.Patterns.Backtrace
       HsBindgen.Patterns.Enum.Bitfield
       HsBindgen.Patterns.Enum.Simple
+      HsBindgen.Patterns.LibC.Arch
   hs-source-dirs:
       src
+  if arch(x86_64)
+    hs-source-dirs:
+      src-x86_64
   build-depends:
     , pretty-show >= 1.10 && < 1.11
     , vector ^>=0.13.2.0

hs-bindgen-patterns/src-x86_64/HsBindgen/Patterns/LibC/Arch.hs

@@ -0,0 +1,2 @@
+module HsBindgen.Patterns.LibC.Arch (
+  ) where