Removed the ValueAST type.

Rather than certain types being based on or requiring a ValueAST as
input, it's up to the AST-using code to define the acceptable relations.

This should make the AST structure more flexible, at the possible cost
of the AST-using code needing a little more type validation.
Support for boolean/logical expressions.

The parser now handles logical operators (&&, ||), relations operators
(<, <=, >, >=, ==, !=), logical negation (!x), and the boolean constants
(true, false).

Also added is parser support for an arbitrary number of arguments in
function calls.
Renamed ANTLR files from Calculator to SimpleLang.
Include some arithmetic in a logical test case.