Allocate $mutable-strings where necessary and implement mutation
procedures.

* module/hoot/compile.scm (utf8->string): Allocate a $mutable-string.
* module/hoot/prelude.scm (string-copy): Allocate a $mutable-string.

(%mutable-string-set-str!): Procedure for setting the contents of a
mutable string.
(string-set!, string-copy!, string-fill!): Implement with string-append
and %mutable-string-set-str!.

* test/test-strings.scm: New tests.