Add default codegen behavior for numeric literals

[leonardt]

Based off vector-decl, so pulled into that branch for the diff.

This changes the numeric literal code gen behavior to leverage the default, implicit syntax for width, signedness, and radix.

[rdaly525]

What is the reason to do this? I think keeping on the radix for 32 bit numbers would be more clear and not too verbose.

[rdaly525]

Oh, actually this seems reasonable especially with slicing.

[leonardt]

@rdaly525 actually, it's mainly motivated by the fact that the coreir primitive definitions rely on "unsized" parameter values. When the default size is explicit, it seems like verilator does a different inference algorithm that triggers a warning. This change unblocks that code. In the future, I think it would make sense to have two modes: one that has implicit default syntax, other that has fully explicit syntax. Then the user could choose which type of numeric literal they'd like to generate (e.g. force a 32'd prefix.