Make ConstantData the mechanism for interacting with constant values

[abrown]

As mentioned in #974 (comment) and #974 (comment), Uimm128 is used for parsing and writing constant values from and to CLIF. If the constant pool is needed for types other than 128-bit vectors, then a more flexible type is needed. ConstantData can become this but needs to:

• become a real type, not a type alias
• acquire implementations for FromStr and Display
• replace Uimm128 in the various places conversions are currently made (e.g. parser.rs, write.rs)

I wonder if in the end this makes Uimm128 superfluous and it can be removed.

[bnjbvr]

Thanks for opening an issue! Sounds great to me.