Allow regular letters as operator names

This is something that has already been discussed. It's also in the list of Commonly Rejected Changes:

Replace logical operators (&&, ||, !, etc.) with words like "and", "or", "not", and allow non-punctuation operators and infix functions: The operator and identifier grammars are intentionally partitioned in Swift, which is a key part of how user-defined overloaded operators are supported. Requiring the compiler to see the "operator" declaration to know how to parse a file would break the ability to be able to parse a Swift file without parsing all of its imports. This has a major negative effect on tooling support. While not needing infix support, not would need operator or keyword status to omit the parentheses as ! can, and not somePredicate() visually binds too loosely compared to !somePredicate().