Unify initializer and imports.

Previously, rust-peg files could have both an initializer (arbitrary code in {} braces) and imports (use statements). The former was only used by the `peg` command-line tool, while the latter was only used by
the `peg!` and `peg_file!` macros.

This removes initializers entirely, and makes the command-line tool use imports. This is removing a feature since the initializer could contain not only imports but also top-level functions and other items, but these
items are better defined in a .rs file and then imported into the grammar anyway.