Support additional typechecker restrictions

This release supports:

• safe_array=true and safe_vector_array=true
• enable_experimental_tc_features=no_fallback_in_namespaces (nightlies, expected in 3.25)

Initial stable release

Code is identical to v0.2 which has been in heavy use for nearly a year; only changes are to documentation

Small improvements

Fixes from @doublecompile - see #1

This makes Subclass::withFoo() return an instance of Subclass, not the superclass that it's defined in.

Initial release - compatible with psr/http-message v1.0

v0.1

make this a composer package