Rework integer literal parser

[AtomCrafty]

Completely rewrote value converters responsible for parsing integer literals as TypedBigInteger instances.
Apart from implementation details, these are the behavioral changes:

• Verilog-style literals now support underscores to separate groups of digits
• Signed Verilog-style literals now provide negative values when the sign bit is set
• Verilog-style literals will now generate diagnostics if the value's bit pattern does not fit within the specified width

Closes #104

[eyck]

Is there a particular reason to switch to List?

[AtomCrafty]

I'm not sure why this was ever Iterator. If anything, it should have been Iterable. We want to return something you can iterate over, not the iterator itself. Unfortunately eAllContents is a single-use iterator, so to make it iterable we need to collect it into a list.