Give a language to a type

This changes main_type to hold a language, and updates the debug
readers to set this field.  This is done by adding the language to the
type-allocator object.

Note that the non-DWARF readers are changed on a "best effort" basis.

This patch also reimplements type::is_array_like to use the type's
language, and it adds a new type::is_string_like as well.  This in
turn lets us change the Python implementation of these methods to
simply defer to the type.