Clean up reference to varray, darray, and HH_FIXME

[lexidor]

fixes #58 fixes #61

Fixes:

• Removes unreachable code UntypedArraySpec. It was private and the array type is impossible to spell (both in the typechecker and in the runtime) anyway.
• Removes all references to darray, varray, and varray_or_darray
  • If a varray or darray type is encountered in TypeSpec\of<T>() or its type structure based friends, they are treated as vec and dict respectively.
  • If a varray_or_darray type is encountered, an UnsupportedTypeException is thrown.
• Removes all HH_FIXME / HH_IGNORE_ERROR directives an replaces them with HH\FIXME\UNSAFE_CAST<Ta, Tb>(...) where needed.

[lexidor]

The tests don't want to run here... You can check https://github.com/lexidor/type-assert/pull/1 for the test output. The push :x: is not meaningful. Something with composer branch aliases. The pull_request :heavy_check_mark: is the one to look at.