[lang-model]: Signed integer types

[archaephyrryx]

We currently have no mechanism, first-class or otherwise, for properly representing unsigned integers of varying widths (i8, i16, i32), which are demanded by various fields of OpenType.

The current approach taken is to mark a parsed unsigned-integer as being 'signed' by wrapping it in a variant, and using twos-complement implicitly to preserve the same bit-sequence as would be used for negative value-ranges, within the unsigned type.

To clarify, the most we need is a generic operator for casting a particular bit-width of unsigned integer to its signed analogue, e.g. Expr::ToSigned{8. 16, 32}(Box<Expr>) that maps U8 -> I8, U16 -> I16, and U32 -> I32 respectively, using the const methods available in Rust for such conversions.