parametric types

To review:

• The T_app case of evaluate_type in src/passes/10-checking/checking.ml
• src/bin/expect_tests/parametric_types.ml
• function t_abstraction* used in src/environment/environment.ml: they now have to have the type_meta field set (e.g. the ast_core version of the type declaration) to be used in checking
• abstractors (some refactoring happenned here and there as well)

all other changes are trivialities

side changes

• remove Layout.convert_* type conversion helper functions as they don't make sense anymore

• has a changelog entry