Turns out that calling `resolve_type_variables_if_possible` in a O(n^2)
loop is a bad idea. Now we just resolve each copy of the region variable
to its lowest name each time (we resolve the region variable to its lowest
name, rather than to its unify-table name to avoid the risk of
the unify-table name changing infinitely many times. That may be
not a problem in practice, but I am not sure of it).
This patch allows type parameter defaults to influence type inference. This is a possible breaking change since it effects the way type inference works and will have different behavior when mixing defaults and literal fallback.
Changes the style guidelines regarding unit tests to recommend using a
sub-module named "tests" instead of "test" for unit tests as "test"
might clash with imports of libtest.
