Allow use of hex literals for uint types

[Philogy]

Simple Summary

Vyper doesn't allow to do the following:

MY_CONSTANT: constant(uint128) = 0xffffffffffffffffffffffffffffffff

Sometimes it's more convenient/clear to use hex as literals for units, disallowing this seems unnecessary/is annoying.

Motivation

Make Vyper easier to use.

Specification

Remove this error and allowing casting of hex literals to uint types.

Backwards Compatibility

Backwards compatible because this was previously disallowed meaning no valid contracts contained this.

Dependencies

None.

References

None.

Copyright

Copyright and related rights waived via CC0

[charles-cooper]

in the 0.3.x series (0.3.0 i think), i disallowed literals from being allowed in multiple type classes. it was to simplify the typechecker code. we could relax that, but just giving you the historical and implementation context for why the rules are currently the way they are.

that being said, having to do stuff convert(0xffff, uint16) is indeed kind of annoying.