Optimize _get_expr_syms().

Instantiating that nested function with free variables turned out to be
quite expensive in cProfile.

For the _build_dep() case, it would be even neater to have a function
like _add_sym_deps(sym, expr) that adds 'sym' to each 'deps' set in
'expr', removing the need for a temporary set. Unfortunately,
_get_expr_syms() is used elsewhere too, and it's probably not worthwhile
having another very similar function just as a small optimization.

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