[mlir:OpAsm] Factor out the common bits of (Op/Dialect)Asm(Parser/Printer)

This has a few benefits:
* It allows for defining parsers/printer code blocks that
  can be shared between operations and attribute/types.
* It removes the weird duplication of generic parser/printer hooks,
  which means that newly added hooks only require touching one class.

Differential Revision: https://reviews.llvm.org/D110375