Return starlarkNodes from the functions that parse them

Currently, mk2rbc is structured around having a global
"receiver" object that accepts all starlarkNodes. As soon
as they are parsed they are added to the receiver.

Returning the parsed nodes to the calling function is more
flexible, as it allows the calling function to restructure
them as necessary. This is the first step to supporting
complicated statements involving $(eval), such as
`$(foreach v,$(MY_LIST),$(eval MY_LIST_2 += $(v)))`

Test: go test
Change-Id: Ia194123cf090d2b9559a25b975ccbc5749357cc0