Add explicitCast helper to std.conv

Makes possible to define more restricted cast that won't accidently
result in broken behavior during refactoring.

As discussed in http://forum.dlang.org/post/m7datc$2p31$1@digitalmars.com
A ddoc question, because I don't know. How will this look? I am worried the docs will go just on the template `castFrom`, and not detail you are supposed to use `castFrom!T.to!U`