Investigate Correct Usage of __SIGNED_INT and __UNSIGNED_INT

[montymxb]

Docs state that __SIGNED_INT and __UNSIGNED_INT are injected under package declarations, as stated on page 190 of the 6.1 lang ref. However, even in the absence of a top-level package declaration, one is apparently synthesized for you:

If the source contains no PACKAGE statement, the
compiler effectively inserts one after the first set of *PROCESS statements...

As such, we've currently added these type aliases as built-ins, since they're available in almost all cases. However it needs to be investigated whether we can construct statements that would reference these types and exist outside a declared (or synthesized package). Lastly it's possible that, despite the documentation, we may be able to reference these injected aliases in such cases. If so we should promote this up to compiler-documentation discrepancy for 6.1