Revert "Remove support for `private`..."

We still need CFCSymbol to support `private` visibility because member
variables need to be implicitly private by default.  It was enough to
remove the keyword from the parser.

This reverts commit 08195486bacb61a64676d7fc837dca051ee1bd5e.
Restrict visibility of member vars.

Disallow exposure specifiers for member vars.  Change the default
exposure from ostensibly `parcel` to `private`.