add early BOZ constant parsing, separate constructor

[raehik]

In the same vein as #193 , we do what we can for BOZs as early as possible.

Thoughts and notes:

• BOZs are now placed in their own constructor, but get parsed identically, which means they are alternatives in the INTEGER_LITERAL rule (in most of the parsers).
• Could do a few more tests.
• No special support for them in the rest of the codebase. If we're to follow the later Fortran standards, BOZ constants are untyped bitstrings, and their behaviour is pretty much all explicit rules like "works like this in this intrinsic function". Could check what fortran-vars deems useful if they have any relevant eval rules!

[raehik]

fortran-vars handles cases where a BOZ constant is used as an integer. I think I'd reconsider putting BOZs as a constructor in an evaluated Fortran value type if it's possible. Instead get all their logic done early, convert them down to typed values.

[raehik]

Discussed. There's more code to consider adapting from fortran-vars for handling BOZs -- getting the AST change out of the way makes future changes easier, so merging.