Start implementing DLL support

Symbols are still stubbed.
Implement DLL symbols
Improve naming
Implement loadext (untested)
Add MVMDLLSym.h to Makefile
Merge branch 'master' into ext
Update headers
Implement extension op registry
Generate dummy labels for extension op cgoto dispatch
Add missing header dependency
Dispatch extension ops
Suggested changes to the bytecode format to support extension ops
Resolve extension ops on validation
Add extension ops entry to bytecode header and fix offsets.

The writer still needs to be adjusted.
Write new bytecode header
Add missing newline
Merge branch 'master' into ext

Conflicts:
	lib/MAST/Ops.nqp
	src/core/interp.c
	src/core/oplabels.h
	src/core/oplist
	src/core/ops.c
	src/core/ops.h
Fix extop validation (hopefully)
Load extension op records from bytecode stream

Operand descriptor validation might need some
changes, in particular for sized constants
which we don't yet support properly.
Disallow ins and callsite operands for extension ops
and fix operand_bytes for lexicals