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
I think this could be better worded. I find it confusing in its current form.

"parameter 'value' is not of specified type '" ~ From.stringof "', it is of type '" ~ T.stringof ~ "'"