[Debt] Clean up serde helpers and remove redundant code

[prestwich]

With the numeric type audit, and #478, we've been maintaining code that is redundant given the switch to ruint from parity-types

this is also followup to #443

• delete U64HexOrDecimal newtype wrapper as redundant
• delete _or_decimal variants of helper modules as redundant
• check other helpers for redundancy
• rename helper modules to u**_via_ruint to make behavior clearer

on consideration, we might want to rename the helpers something like u64_via_ruint to clarify behavior

Originally posted by @prestwich in #478 (comment)