Merge VarRef into ExprVar
Remove some unused functions
Show the exact position of undefined variables

In particular, undefined variable errors in a "with" previously didn't
show *any* position information, so this should help a lot in those
cases.
Treat undefined variable errors consistently

Previously, a undefined variable inside a "with" caused an EvalError
(which can be caught), while outside, it caused a ParseError (which
cannot be caught).  Now both cause an UndefinedVarError (which cannot
be caught).
Deduplicate filenames in Pos

This saves ~4 MiB of RAM for NixOS system instantiation, and ~18 MiB
for "nix-env -qa".
printStats(): Print the size of the symbol table in bytes