Adds new module `string` to be later converted to its own
crate. The module currently contains the FromHexStr trait and an error
type to be used for implementing hex parsing on types. This change
also adds implementations of FromHexStr for types with a single u32
member such as `Sequence(pub u32)`. All structs that match the
following regex have been given this implementation
`\(u32\)` and `\(pub u32\)`. All implementations have associated
unit tests matching all possible cases. NonStandardSighashType has
been ommitted from this change as it is an error and should not be
constructed using the methods added in this change.
Adds parse::hex_u32 for future use to be made generic to allow
different sizes of integers to be parsed from hex strings.
The error type FromHexError implements required traits such as
Display and std::error::Error
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