use ffp_parse_string instead of shelling out to hh_parse

[ssandler]

This yielded a dramatic speedup in HHAST for us, and as far as I can tell is absolutely equivalent.

Since the built-in type of ParserCache is opaque, I had to as dict<_, _> it, and to avoid adding FIXMEs I changed the type annotations where it's used to allow arraykey keys.

The built-in parser doesn't seem to support old style xhp children declarations, but since those have been deprecated for several years I think it's ok to remove support for them in HHAST and the associated migration, so I've done that as well.

[lexidor]

hhast/src/entrypoints.hack

Line 17 in 34797e7

vec<string> $user_args = vec[],

The $user_args argument is not used. This means there is no need to bypass the cache when not empty anymore.

Alternatively, keep the old implementation around and start this function with if (!C\is_empty($user_args)) return await from_file_hh_parse_async(...); and consider HH\ffp_parse_string() as a fast path.

[ssandler]

@lexidor good point! I'm incorporating that and also working on fixing CI.

[ssandler]

CI is broken for other reasons, moved the work to get that working into #538