Make OP_METHOD* to be of new class METHOP

Introduce a new opcode class, METHOP, which will hold class/method related
info needed at runtime to improve performance of class/object method
calls, then change OP_METHOD and OP_METHOD_NAMED from being UNOP/SVOP to
being METHOP.

Note that because OP_METHOD is a UNOP with an op_first, while
OP_METHOD_NAMED is an SVOP, the first field of the METHOP structure
is a union holding either op_first or op_sv. This was seen as less messy
than having to introduce two new op classes.

The new op class's character is '.'

Nothing has changed in functionality and/or performance by this commit.
It just introduces new structure which will be extended with extra
fields and used in later commits.

Added METHOP constructors:
- newMETHOP() for method ops with dynamic method names.
  The only optype for this op is OP_METHOD.
- newMETHOP_named() for method ops with constant method names.
  Optypes for this op are: OP_METHOD_NAMED (currently) and (later)
  OP_METHOD_SUPER, OP_METHOD_REDIR, OP_METHOD_NEXT, OP_METHOD_NEXTCAN,
  OP_METHOD_MAYBENEXT

(This commit includes fixups by davem)