(PUP-1579) Rename the Literal type to Scalar

Calling the type "Literal" was bad because it is used not only
for literal values entered in source code, but naturally also
for computed values. A commonly used name for these types is "Scalar".

This commit changes "Literal" to "Scalar" everywhere:
* class name PLiteralType => PScalarType
* its label
* factory methods "literal" => "scalar"