Started implementing splats. Splat in def arguments are now working. Missing: combine them with default values and also allow splatting call arguments.

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)

Moved some core specs to codegen so that we don't need to compiler the compiler and then run the specs to check if everything still works

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)`.

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