Hardcode array_map typing into typechecker - argument typing

Summary: As in title, see comment in .hhi file and unit tests for explanation.

Reviewed By: @elgenie

Differential Revision: D1566676
Fix in_array->array_key_exists conversion

Summary: It needs a static array, not a smart-allocated array with
the static bit set.

This also ensures that we only get one copy of each distinct array.

Reviewed By: @swtaarrs

Differential Revision: D1602742
Remove some helpers in datatype.h

Summary: One isn't used and one had a single caller.

Reviewed By: @mxw

Differential Revision: D1597974
Remove some MIN/MAX macro uses in the emitter

Summary: <algorithm> has preferable type-safe versions that don't double-eval
their args.

Reviewed By: @paulbiss

Differential Revision: D1599803
Remove now-unneeded MAP_ANON defines in read-only-arena.cpp

Summary: This should've gone away when it was switch to use malloc instead of
mmap.

Reviewed By: @bertmaher

Differential Revision: D1601873
Use smaller HHBC arena chunks by default

Summary: Why so big

Reviewed By: @bertmaher

Differential Revision: D1601886