Complete identifiers following non-whitespace

Now, identifiers following non-whitespace, non-identifier characters (such as
operators or parentheses) will also be completed successfully.
Fix comedy error in termination checker
Only use metavariables as completions for :p and :prove
Stop completion of names that are prefixes of others

Previously, when completing "plus", it was accepted. Now, it tries completing
with "plusAssociative", "plusCommutative", etc, and the user has to press
space.