(perf) Make small performance improvement to assignment validation

It is faster to pass an explicit number of arguments
(maint) Fix issue after optimizing validation calls; renamed var

Instead of passing *args for a single argumet it was made explicit.
When refactoring, the rename was missed in one place. This corrects the
mistake.
(maint) Remove unused model class LiteralText

This class was never used since LiteralString works just fine eveywhere.
The distinction that LiteralText used different escapes is a moot
point, as after parsing has taken place it does not really matter.
(#22363) Refactor model of literals

There were several problems with the model of literals:
* LiteralNumber had radix which is not applicable for floats
* All LiteralValue used an inherited Object attribute which makes it
  difficult/impossible to correctly serialize literal values.

The refactoring moves the value attribute to each subclass, adds
LiteralInteger (with radix), and LiteralFloat as subclasses of
LiteralNumber. 

LiteralRegexp required special handling since a regular expression
must be serialized as a String. Now LiteralRegularExpression has two
attributes 'pattern' (string), and 'value' (regexp). Setting either
changes the other. When serialized, only the pattern is serialized (and
the regexp value is reconstructed on de-serialization).