This parser builds a radix tree from non empty string alternatives.
Radix trees are interesting as they allow for efficient parsing of
string alternatives (e.g. enum names) without backtracking and require
only 1 character lookahead.

String alternatives are first sorted and then grouped by common non
empty prefix. Grouped string are then recursively grouped with the same
process. In the resulting tree, each group of substrings start with a
different character, which can be used to decide which alternative
branch to take.

This operator is inspired by fastparse `stringIn` one, and requires all
alternatives to be non empty.