std.conv: clean imports

fix parse for null and bool

1. Allow parse nulls from input range
2. Remove slow (unicode like) version of parse for bools.
3. Range version of parse for bools is used.

revert some changes

remove some changes

remove few changes
added scope format imports
Merge pull request #2667 from 9il/conv

std.conv: clean imports & parse (null and bool) fix