This PR splits the environment used by the kernel from that used by the
elaborator, providing the foundation for tracking of asynchronously
elaborated declarations, which will exist as a concept only in the
latter.
Minor changes:
* kernel diagnostics are moved from an environment extension to a direct
environment as they are the only extension used directly by the kernel
* `initQuot` is moved from an environment header field to a direct
environment as it is the only header field used by the kernel; this also
makes the remaining header immutable after import
This PR modifies structure instance notation and `where` notation to use
the same notation for fields. Structure instance notation now admits
binders, type ascriptions, and equations, and `where` notation admits
full structure lvals. Examples of these for structure instance notation:
```lean
structure PosFun where
f : Nat → Nat
pos : ∀ n, 0 < f n
def p : PosFun :=
{ f n := n + 1
pos := by simp }
def p' : PosFun :=
{ f | 0 => 1
| n + 1 => n + 1
pos := by rintro (_|_) <;> simp }
```
Just like for the structure `where` notation, a field `f x y z : ty :=
val` expands to `f := fun x y z => (val : ty)`. The type ascription is
optional.
The PR also is setting things up for future expansion. Pending some
discussion, in the future structure/`where` notation could have have
embedded `where` clauses; rather than `{ a := { x := 1, y := z } }` one
could write `{ a where x := 1; y := z }`.
Following up #5928, updates the syntax for `omega` and `solve_by_elim`
and restores the syntax quotations in their implementations.
Following up #5898, uses the new tactic syntax in the library, replacing
all uses of `(config := ...)`.
As far as I can tell, the ability to pass a structure instance to a
deriving handler is not actually used in practice. It didn't seem to be
used in the test suite, at least.
Do we want to remove this, or do we want to use and document it? This PR
removes it, but that's not something I feel strongly about - but seeing
if it breaks Mathlib is a useful data point.
