Converted libcore/int-template.rs to the new string functions.

- Moved ToStr implementation of integers to int-template.rs
- Marked the `str()` function as deprecated.
- Forwarded all conversion functions to `core::num::to_str_common()`
  and `core::num::from_str_common()`
- Fixed the places in the codebase where `to_str()` is being used.