"writable" is not in use and probably we can go in the reverse direction
and remove the flags that are useful for enabled writable, this is just
for the sake of completeness however.

It removes io.h and other polyfills which we no longer need as 7e76d746

This makes it compatible with ARMCC which I had access in
a collaboration with @imgtec, thanks!

Basically hb_blob_create_from_file features three code paths,
mmap, Win32 and fallback.

We had fallback implementation even before this but it was relied
to "open" which is not available on some environments. This change
improved the situtation by using only fopen and friends for
fallback path.

Interestingly we could use "open" on Windows but in fact it was
emulated by MSVCRT so I've completely split that from Unix path
now that we have a distinct path for fallback path also.

Seems like I messed up; buffer overrun got reported.

Following to 63f57f4d

Trying to see how many can converted meaningfully.

[subset] Add cmap format 4 subsetting.

This saves each object allocation 72 bytes.  Now object overhead is just
16 bytes (on x86) instead of 88 bytes. Neat.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1023

Right now only Old Italic is marked as such.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1000

Towards making blob more memory-allocation-friendly

We should hang those off somewhere else. For now, the unused ones can go.

To avoid need of CMP0063 which is not available on older CMake versions

[subset] Switch to a non-log using implementation of caculating searchRangeZ, entrySelectorZ, and rangeShiftZ in cmap4.