Merge pull request #2060 from ericmj/v0.13

Add `System.delete_env/1`
Update Macro functions to support maps
Do not accumulate results in comprehensions if they won't be used
Support end-of-line backslash in quoted expressions

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Add try/case/receive to list of constructs that can handle returns
Add _ = EXPR to the list of expressions that can handle no return