mux: merge path and wildcard types
mux: create separate path and wildcard types

Previously the path type acted as a wildcard and could only exist as a
leaf in the routing tree.
Now there are separate path and wild types, where wild behaves the same
way path did before and path is like string except that its value is the
remainder of the path (which still must be matched itself).
mux: update Handler docs for the newer behavior
mux: add test for request canonicalization
mux: fix code in package level docs
mux: add issue tracker link in readme
mux: don't embed node in ServeMux
mux: move options into own file
mux: simplify test code
.builds: add SourceHut CI config
mux: fix options handling bug and add more tests
mux: support sending 405 Method Not Allowed
mux: fix minor linter issues
mux: change name of Options option and test it
mux: reduce code duplication in tests
mux: don't support host based matching
mux: be careful about complying with Go's license
mux: expand docs and add examples
mux: expand tests for route registration panics
mux: support OPTIONS requests by default
mux: panic if routes are unclean

Also remove some panic constants that weren't necessary