Not functional change (expected!).

This makes a lot of code safer.  We only try modifying the object in one
place, after making sure it's safe to do so.  So, do a const_cast<> in
that one place...

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.

Avoids USHORT, SHORT, ULONG, LONG clashes with Windows API.

Should fix warnings generated when building with -Wunused-private-field.
Based on patch from Jonathan Kew.

More to come, for APPLY, CLOSURE, etc.

Where causing issues for people with MSVC.

Not used for anything right now.  Will use to get num_glyphs in the future.

Step version up to 0.5.0.

Also, fix to pass "make distcheck"

Also optimizes the common-direction script lookup.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.