* Fixes a panic with defining two commands with the same name caused by
declaration not found after predeclaration.
* Adds a new error if a custom command is defined more than once in one
block.
* Add some tests
* Hiding logic is simplified and fixed so you can hide and unhide the
same def repeatedly.
* Separates predeclared ids into its own data structure to protect them
from hiding. Otherwise, you could hide the predeclared variable and
the actual def would panic.
