Change char::escape_{default,unicode} to take callbacks instead of allocating
strings
Specialize to_str_common for floats/integers in strconv

This allows the integral paths to avoid allocations on the heap

Closes #4424, #4423
auto merge of #7465 : alexcrichton/rust/issue-4432, r=graydon

This stems from trying to perform as few allocations as possible throughout the standard libraries.

This specializes the `ToStr` implementation for floats/ints separately because it's known that ints will have a maximum length (whereas floats could be very very large).

I also removed a `FIXME` to remove a malloc from the `to_str()` of floats in `repr.rs` because I think that this should be addressed elsewhere. I think that we may not be able to avoid it easily because floats can have such large representations, but regardless this should be a problem with the implementation of `float_to_str_bytes_common` now and not in the `Repr` module.