lib/uri: performance improvements [misc #10628]

* lib/uri/generic.rb (split_userinfo): fstring for 1-byte split
  (set_port): reduce bytecode size
  (check_path): reduce garbage via opt_str_freeze
  (query=): ditto
  (fragment=): ditto
  [misc #10628]
* lib/uri/rfc3986_parser.rb (regexp): cache as attr
  (initialize): setup and freeze regexp attr once
  (split): reduce bytecode size, use opt_str_freeze
  (parse): minor bytecode and garbage reduction
  (default_regexp): rename for initialize

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48980 b2dd03c8-39d4-4d8f-98ff-823fe69b080e