Make locs an explicit type rather than abbreviation for a pair

This should make automatic tools treat values of the type as atomic
rather than split them apart, which is usually unhelpful.

Define valid_lptree constant (from CakeML).

This constant asserts that the parse tree respects the provided
grammar (as per valid_ptree), but also that every non-terminal node in
the tree has the correct locations recorded (derived from the
locations of the sub-trees, using the merge_list_locs constant.

Cleaner approach of location in parser

location datatype definition