Handle map fields in their own function in erl_eval

Map fields (K := V, K => V) are not expressions and shouldn't be clauses of
erl_eval:expr/5.
Support maps in erl_prettypr
stdlib: Add tests for Maps in erl_prettypr
Merge branch 'nox/maps-improve-erl_eval'

* nox/maps-improve-erl_eval:
  Handle map fields in their own function in erl_eval
Merge branch 'nox/maps-erl_prettypr'

* nox/maps-erl_prettypr:
  stdlib: Add tests for Maps in erl_prettypr
  Support maps in erl_prettypr