move parser to diagnostics

this moves all errors in the parser to diagnostics. see #190

pipeline context

a94c4580 is interesting: the following code panics on master

fn top() -> bool {
    let x = true;
    stage(+1).x
}

thread 'main' panicked at spade-ast-lowering/src/lib.rs:1511:18:
Expected to have a pipeline context

this is now reported with a diagnostic

error: pipeline stage reference in function
  ┌─ main.spade:3:5
  │
1 │ fn top() -> bool {
  │ -- this is a function
2 │     let x = true;
3 │     stage(+1).x
  │     ^^^^^ pipeline stage reference not allowed here
  │
  = note: only pipelines can contain pipeline stage references

Error: aborting due to previous error

Author checklist

• New Diagnostics have at least one snapshot test that triggers it
• [n/a] Added a line to CHANGELOG.md, if relevant