parametric arguments that are not used in fields are forgotten before they're needed

**Describe the bug**

```
pub struct SomeType { data: uN[FIRST] }

pub fn dostuff
(a: SomeType) -> SomeType {
a
}

#[test]
fn test_dostuff() {
let a = SomeType { data: u4:0 };
let result = dostuff(a);
assert_eq(result.data, u4:0);
}
```
error:
```
0113: pub fn dostuff
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^----^ TypeInferenceError: Parametric expression `SECOND` referred to `SECOND` which is not present in the parametric environment;
```

**To Reproduce**
Copy above and build.

**Expected behavior**
I expect this to compile without trouble, and that `COMPUTED` is a parametric argument equal to `SECOND` (which I would then use later in the parametric section or body of the function).

**Additional context**
There appears to be too-early parametric type erasure. Any parametric types not used to define a field of a struct are erased before function parametric type inference occurs.

This problem appears to be caused by the fact that SECOND is not used to define a field in `struct SomeType`. This can be demonstrated two ways:

1. Change: `COMPUTED: u32 = {FIRST}`. This compiles (because FIRST is used by `data`). Now change `data: uN[SECOND]` and it fails to compile (because FIRST is no longer used by `data`.)
2. (don't do # 1 before doing:) Add another field to the struct, and define the field using `SECOND`, e.g. like `unused: uN[SECOND]`. Resolve the compile errors that result from this new field. Now it compiles.