090dad770f Improve string parsing (Tobin C. Harding)
Pull request description:
Currently we implement string parsing for height/time from the `absolute` module but not the `relative` module.
Improve the macros used to implement string parsing and use the new versions to implement string parsing for the height and time types in `relative`.
Done while reviewing data structures in relation to `serde`.
ACKs for top commit:
apoelstra:
ACK 090dad770f
Kixunil:
ACK 090dad770f
Tree-SHA512: bfa88efbaf5dc35755eb46df373a08e223f112860e8a65f58db9fdd77e2c01dc9377da735b33ef58940004fe5fe11690ac09be19591fded2c9fd04cd7d2bdf73