Inline proper_types:is_raw_type/1 the hottest function

It speeds up usage within module a little bit.
Inline proper_gen:clean_instance/1 the second hottest function

It speeds up usage within module a little bit.
Simplify proper_statem:execute/4 to /3 (internal anyway)
Simplify keep_shrinking/3 to /2.
Simplify proper_arith:safe_map/2.
Simplify proper_arith:safe_zip/2.
Improve efficiency of proper_arith:partition/2 and proper_arith:filter/2

Traverse list in reverse order to avoid four or two reversing at the end.

Build only required lists for filter.
Simplify proper_arith:{remove/2, insert/3, unflatten/2}.
Inline some hot and short functions in proper_types module.