@Kha The default value (false) for `pp.inaccessibleNames == false` help when
visualizing error messages (see test
`hidingInaccessibleNames.lean`). We added this feature after to hide
intermediate variables created by `match_syntax`.
However, this default value confused me in tactic mode. For example,
it will hide a hypotheses `x : Fin 0` if nobody depends on it, but as
a user we want to know we have it since we can close the goal using
it. Thus, I added `withPPInaccessibleNames act`, it executes `act`
using `pp.inaccessibleNames true` if the user did not explicitly set
it. I use this combinator at `FileWorker` and when producing the
`unsolved goals` error message. In all other scenarios, I believe
hiding these inaccessible variables is a good thing.
@Kha We will probably have to refine the heuristic for hiding the
inaccessible names, but the first version is already useful.
Here is the error message for a `_` before this commit
```
error: don't know how to synthesize placeholder
context:
x✝⁴ : List Nat
x✝³ : Nat
x✝² : x✝⁴ ≠ []
a b x✝¹ : Nat
x✝ : [a, b] ≠ []
⊢ Nat
```
After
```
error: don't know how to synthesize placeholder
a b : Nat
: [a, b] ≠ []
⊢ Nat
```
We need `MetaM` methods such as `isProp` to improve `ppGoal`.
This commit also moves `currNamespace` and `openDecls` to
`Core.Context`. Without this change, `Meta.ppExpr` was not taking
`open` commands into account.
