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.

The message previously made reference to the now-removed `access`
attribute.

The mix builtin is defined with the inverse order for it's arguments
compared to the IR when the selector is a boolean vector.

Glsl defines that if a matrix constructor is called with only a scalar
it's result will be the matrix where the scalar is used if the row and
column are equal and everywhere else a zero

Glsl allows things such as

```glsl
layout (location = 0) in vec4 a;
vec4 b = a;
```

In this case `b` depends on `a` which can't be known at compile time,
so it's necessary to add a small prologue to the entry point to
initialize it.