Refactor convert / cast functions

[chaoticlonghair]

• Use TryFrom / TryInto to convert structs

  Remove:

  • https://github.com/cryptape/rust-numext/blob/33990f62c7c1558d0e8aacb4bfc461e60d6e53dc/constructor/src/fixed_uint/core/internal/kernel.rs#L162-L165
  • https://github.com/cryptape/rust-numext/blob/33990f62c7c1558d0e8aacb4bfc461e60d6e53dc/constructor/src/fixed_hash/core/internal/kernel.rs#L141-L144

• Use TryFrom / TryInto to convert between fixed uints and primitive unsigned integer types.

  For example, cast a H256 into u64. (At present, no function can do this directly.)