Remove/Rename un-namespaced macros

[klange]

We expose a lot of macros which are not prefixed with KRK_ or krk_. To provide a clean API, these should be renamed.

• *_VAL macros for creating value cells. These should be renamed to look like function calls and use the correct type names, eg. krk_new_int or something similar.
• AS_* for extracting from value cells and objects. These should at least be prefixed, and the value ones should be updated to use the right names. krk_as_?
• IS_* type checking convenience macros. Same as above.
• Various old-style argument checking macros like METHOD_TAKES_*/FUNCTION_TAKES_*.
• Binding macros, like BIND_METHOD, and the related MAKE_CLASS macro (which doesn't seem to be used?)

While we're at it, removing the old-style KRK_FUNC/KRK_METHOD macros would be worthwhile. All of the code I have on hand has been converted to the new-style KRK_Function/KRK_Method macros.