The C++ templates were hard to understand and modify. Instead of using
the templates, let's monomorphize by hand and generate equivalent
classes for each FFI function signature.
I tried a slightly different approach for the template code. . In the
template struct itself, we now generate some simple Rust structs with
all the data the templates need. I like this because it's easier to
compute these values in Rust than in an Aksama template.
Differential Revision: https://phabricator.services.mozilla.com/D212342
2f26a4bc8c