Introduce hierarchical type system and refactor operator / implementation registration

[finn-rudolph]

There are quite a few issues I have to fix, but some tests already run again. (In particular, the signature tree must be able to deal with template types.)

Changes:

• The operators are now instances, not classes. This makes configuring them for code generation a bit less clumsy.
• Implementation registration is now done within a single with statement, not a separate one for each operator
• There is a implicit conversion cost matrix in tree/types.py called IMPLICIT_CONV. We may have to change this to make all the conversions behave as expected. The cost of a conversion is a tuple of two ints.
• All signature-related stuff is not done via strings + parsing anymore, but via actual type objects.