We have now again two functions called h_platform_vasprintf and
h_platform_asprintf. On windows they are implemented in terms of vsnprint
and the like. On BSD/GNU libraries we use the supplied vasprintf and
asprintf.

Since the block of code has been disabled anyway and is not very
portable, we introduce a define to control both the include
and the code.

Storing the size of the blocks is now only activated when DEBUG__MEMFILL
has been defined.

We replace instances of void* w/ char* arithmetics.

This allows system_allocator.c to be compiled with MSVC.

Also we introduce a struct to represent the size_t header.

We use QueryPerformanceCounter which will return realtime, not user
time.

We now have a `stopwatch` to measure time between two moments.

This BSD call will have to be implemented on windows. Right now
we have a stub that just exits the process.

We wrap all remaining references to __attribute__ into a macro call to
H_GCC_ATTRIBUTE, to allow compilation with MSVC CL.EXE

We substitute H_GCC_ATTRIBUTE to any occurence of __attribute__ and
disable its function with non gcc, non clang compilers.

We use a H_ macro so as to not pollute users' namespaces.

On windows build without c4201 so as to allow anonymous struct. This
shows hammer is not C99 compliant but rather C11.

The pretty printing format chosen is USER:<token name>

We use the registry function `h_get_token_type_name` which returns 0
when no name has been defined. This will print (null) as a token name.

NEWS:
* Switching endianness mid-byte no longer potentially re-reads bytes.
* bit_offset now consistently refers to the number of bits already
  read.
* HParsedTokens now have a bit_length field; this is a size_t.  This
  may be removed for memory reasons.

The bit writer has not yet been updated to match; the result of
switching bit writer endianness in the middle of a byte remains
undefined.

I've found this especially useful in combination with my own
_attr_uint_const for things like flags and type specifiers.

It's possible that its usefulness might be diminished significantly if
there were a built-in bitfield constant parser -- that certainly would
eliminate all of my current uses of it -- but it still seems nicely
symmetric with H_ACT_APPLY.