This PR resolves an issue where the `Meta.Context.configKey` field is
private but we still want to use the constructor of the structure for
setting other fields, which would be prevented by the module system
checks:
```lean
structure Context where
private config : Config := {}
private configKey : UInt64 := config.toKey
...
def ContextInfo.runMetaM (info : ContextInfo) (lctx : LocalContext) (x : MetaM α) : IO α := do
-- cannot call private constructor of `Meta.Context`!
(·.1) <$> info.runCoreM (x.run { lctx := lctx } { mctx := info.mctx })
```
Instead, the private field is extracted into an (existing) structure
that applies its default value:
```lean
/-- Configuration with key produced by `Config.toKey`. -/
structure ConfigWithKey where
private mk ::
config : Config := {}
key : UInt64 := config.toKey
structure Context where
keyedConfig : ConfigWithKey := default
```
Thus `Context`'s constructor remains public without exposing a way to
set `key` directly.
ddc4cf0a97