fmt now automatically prints the numeric value of an enum class member
by default, so we don't need to use casts any more.
Reduces the line noise a bit.
These are std::shared_ptr instances underneath the hood, which means
copying them isn't as cheap as a regular pointer. Particularly so on
weakly-ordered systems.
This avoids atomic reference count increments and decrements where they
aren't necessary for the core set of operations.
Instead of having a vector of unique_ptr stored in a vector and
returning star pointers to this, use shared_ptr. While changing
initialization code, move it to a separate file when possible.
This is a first step to allow code analysis and node generation beyond
the ShaderIR class.
