Add .insert() and .insert_char() methods to ~str.
Abort on double-failure. #910

Previously this was an rtabort!, indicating a runtime bug. Promote
this to a more intentional abort and print a (slightly) more
informative error message.

Can't test this sense our test suite can't handle an abort exit.
Change rmake and doc-test to support TESTNAME

Closes #11288
Closes #11222
Fix windows dist script

In copying the license for the third-party bins it is attempting
to delete a directory that doesn't exist.
libsyntax: De-`@mut` `ParseSess::included_mod_stack`
librustc: De-`@mut` the parse session
libsyntax: De-`@mut` `StringReader::pos`
libsyntax: De-`@mut` `StringReader::last_pos`
libsyntax: De-`@mut` `StringReader::col`
libsyntax: De-`@mut` `StringReader::curr`
libsyntax: De-`@mut` `StringReader::peek_tok`
libsyntax: De-`@mut` `StringReader::peek_span`
libsyntax: De-`@mut` `TtReader::interpolations`
libsyntax: De-`@mut` `TtReader::repeat_idx`
libsyntax: De-`@mut` `TtReader::repeat_len`
libsyntax: De-`@mut` `TtReader::cur_tok`
libsyntax: De-`@mut` `TtReader::cur_span`
libsyntax: De-`@mut` `TtReader::stack`
libsyntax: De-`@mut` `StringReader`, `TtReader`, and `reader`
librustc: De-`@mut` the span handler