draft `StringBuilder` interface to encode mutable string operations

[gefjon]

It's Java's StringBuilder. But in Coalton.

After thinking about #635 , I realized that it would be nice to have a way for the library to implement efficient string-wrangling operations like join without allocating and copying a bunch of intermediate strings. We could write all those algorithms in lisp blocks, or we could chicken out and offer mutable operations on String, but I think I'd rather have a type to encode mutable strings, and pretend String is immutable.

Open questions:

• What operations do we want to offer on StringBuilder? I wrote just push-char! and push-str!, but it seems sensible at the very least to offer capitalize!, upcase! and downcase!, since CL makes them super easy for us. There are probably others I'm forgetting, too.
  • I'd prefer not to offer operations for which (vector character) has poor performance as an implementation of strings, like left-trim, push-front or insert-at. The point of StringBuilder is to be an efficiency hack, so I think it should only offer operations for which it's actually efficient.
• Should this be its own package, or a part of coalton-library/string? The latter is appealing because it would make it possible to define string operations (like join) in that package in terms of StringBuilder, where currently I fear we'd have to have another package downstream of both. Or we could move StringBuilder earlier in the library system so that string.lisp depends on it instead of vice-versa.