The new api allows for reuse while keeping some allocations and to
please the borrow checker in future work, it also splits the parser into
logical modules to make it easier to work on.
WGSL will require this. Note that this still might
cause some issues with multi-entry-point GLSL
that I didn't know how to handle. That is, we will
handle unused builtin inputs but not unused
builtin outputs correctly right now. This is an existing
issue though, not a regression.
This is also provided as an option, but I feel like
the more correct approach is to never strip linkage
variables. We'll see though.
