Add TypeParameter (RedHat’s TypeParameterDeclaration)

I decided against having a TypeSpecifier node because the syntax "="Type
is only used in one other place (import aliases), and there the grouping
is the other way around (the "=" belongs to the LHS alias, not the RHS
name, whereas here it belongs to the RHS default argument, not the LHS
name).
Add tests for TypeParameter backend conversion
Add TypeArguments node (remove alias) for #50

Yes, this is definitely the way to go. There was lots of copied+pasted
code that this removes.
Add TypeParameters (RedHat’s TypeParameterList)
Add tests for TypeParameters backend conversion