Avoid the compile a ffi propdef more than once

See AMethPropdef::compile_ffi_method

FIXME find a better way

Property definitions

nitc :: light_ffi $ MModule :: compiled_ffi_methods=
	# Avoid the compile a ffi propdef more than once
	# See `AMethPropdef::compile_ffi_method`
	# FIXME find a better way
	private var compiled_ffi_methods = new HashSet[AMethPropdef]
src/ffi/light_ffi.nit:66,2--69,61