Allow Macro.escape([1|2])

This fixes #1646
Merge pull request #1648 from pminten/quote-improper-lists

Allow Macro.escape([1|2])
Allow any function identifier as a variable, closes #1649
Remove empty Tuple module
Add Enum.slice/3
Merge pull request #1650 from ericmj/enum-slice

Add Enum.slice/3
Update CHANGELOG
Add a count guard check for slice
Implement `Stream.flat_map/2`

Fixes #1642
Use ifnone in Enum.find for all collections
Merge pull request #1654 from ericmj/fix-enum-find

Use ifnone in Enum.find for all collections
Merge pull request #1652 from vanstee/stream-flat-map

Implement `Stream.flat_map/2`