inverse of @uri

[pkoppstein]

@uri does a nice job with multi-byte characters, e.g.:

"á" | @uri' -> "%C3%A1"

It seems that currently there is no support for the inverse of @uri. Since the inverse is quite tricky to code (as illustrated by the above), it would be helpful to have a builtin filter for performing the inverse operation.

[dtolnay]

jvp_hex_next from #373 may be useful for this.

[jcracknell]

I needed an implementation of this:
https://gist.github.com/jcracknell/52cf4e0f8c4518a853784638db8a258d