Add support for `string-set!` and `string-fill!`

[acieroid]

This would require store-allocating strings, and may have an impact on the size of the store, as well as on precision.

This is required for benchmarks such as gambit/compiler.scm, gambit/scheme.scm, and gambit/slatex.scm

[noahvanes]

Strings are now store-allocated, and primitives string-set! and string-fill! have been added.

Caveat: the current string-set! implementation is sound, but not always the most precise (cf. #17)

Notes about the mentioned benchmark programs:

• benchmark gambit/compiler.scm won't work in the concrete interpreter yet, due to lacking support of call/cc (see #18)
• benchmark gambit/scheme.scm seems to be broken for other reasons
• benchmark gambit/slatex.scm requires further IO configuration to run properly