Support for explicit cmdletys arguments with colon

These commits allow parameters to use the ":" at the end to explicitly require the next term to be its argument.
It for example allows SwitchParameters to being switched off explicitly by using `-Switch:$false`.

Also, LanguagePrimitives now correctly converts to SwitchParameter with the correct IsPresent value.
Moved LangaugePrimitiveTests to ReferenceTests.