Added is_control() function, method, tests, and documentation.

In "/src/libstd/char.rs", there are function and method definitions for `is_lowercase()`, `is_uppercase()`, `is_whitespace()`, etc. However, there was no function or method for control characters, so I added the `is_control()` function and method definitions along with documentation and tests. Running `./configure && make check` shows that all tests for `is_control()` pass.