[WIP] Simplify type printing in text format

[tlively]

Instead of reimplementing type printing in Print.cpp, use the new type printing API provided by wasm-type.h. Also change from generating complicated type names that try to convey type structure but that in practice are not very readable to generating simple type names.

WIP because there are still some tests that need to be manually updated, and also because I will probably want to separate the code structure change and the output format change into separate changes. Comments on the general direction very welcome, though.