This PR removes most cases where an error message explained that it was
"probably due to metavariables," giving more explanation and a hint.
## Example
```
def square x := x * x
```
Before:
```lean4
typeclass instance problem is stuck, it is often due to metavariables
HMul ?m.9 ?m.9 (?m.3 x)
```
After:
```
typeclass instance problem is stuck
HMul ?m.9 ?m.9 (?m.3 x)
Note: Lean will not try to resolve this typeclass instance problem because the
first and second type arguments to `HMul` are metavariables. These arguments
must be fully determined before Lean will try to resolve the typeclass.
Hint: Adding type annotations and supplying implicit arguments to functions
can give Lean more information for typeclass resolution. For example, if you
have a variable `x` that you intend to be a `Nat`, but Lean reports it as
having an unresolved type like `?m`, replacing `x` with `(x : Nat)` can get
typeclass resolution un-stuck.
```
In addition to providing beginner-and-intermediate-friendly explanation
about **why** typeclass instance problems are treated as "stuck" when
metavariables appear in output positions, this PR provides
potentially-valuable improvement even to expert users: it explains
**which of the typeclass arguments are inputs** and therefore need to be
fully specified before typeclass resolution will be attempted. This
information can be tricky to find otherwise.
## Next steps, but probably after this PR
* error explanation
* detecting when the syntactic source is a binop and giving a
special-cased explanation on the binary operators and their associated
typeclasses
* detecting when the syntactic source is a function call, inspecting the
function call's type somewhat, and replacing the generic "replace `x`
with `(x : Nat)` hint with a specialized "replace `foo` with `foo (tyArg
:= Nat)`" hint
3f4e85413e