`Sequence` didn't have `FromStr` nor `TryFrom<{stringly type}>`
implemented by accident. This moves a macro for implementing them from
`locktime` module to the `parse` module, renames it for clarity and uses
it to implement parsing for `Sequence`.
When debugging parsing errors it's very useful to know some context:
what the input was and what integer type was parsed. `ParseIntError`
from `core` doesn't contain this information.
In this commit a custom `ParseIntError` type is crated that carries the
one from `core` as well as additional information. Its `Display`
implementation displays this additional information as a well-formed
English sentence to aid users with understanding the problem. A helper
function parses any integer type from common string types returning the
new `ParseIntError` type on error.
To clean up the error code a bit some new macros are added and used.
New modules are added to organize the types, functions and macros.
Closes#1113
Martin Habovstiak