Added `\u` unicode escape sequences inside strings and chars (similar to Ruby). Added `String#chars`.

Fixed #154 and other constant-related bugs. Also: removed the "feature" of regex literals being automatically assigned to a constant to avoid multiple recompilations of the same regex (users must be able to choose that to)

Macro defs are now written like `macro def name(args) : ReturnType` instead of `def name(args) : ReturnType`, which was a bit confusing. Partially related to #160

Allow implicit conversion to C types by defining a `to_unsafe` method. This removed the hardcoded rule for converting a `String` to `UInt8*` and also allows passing an `Array(T)` to an argument expecting `Pointer(T)`.

Lexer: check that integer literals fit their type. Also: deduce their type to Int32, Int64 or UInt64 depending on their value.

Replaced `@name` with `@class_name` inside macros. And: instance vars inside macros don't have the `@` in their name anymore.