Fix trailing garbage syntax bug

[jnu]

The crocodsl grammar only defined partial expressions, without any rule to parse a complete input in one go. Because of this, calling parser.expr() on any input would simply stop without error when a full expression was parsed even if the input token stream was not fully consumed. This led to the following situation:

Input expression: TimeSince($assigned, 'mo') Gte 6
Parse result:     TimeSince($assigned, 'mo')

The Gte 6 piece was unrecognized by the expr method so it was simply ignored, since the LHS parsed correctly. The supposed function Gte would have caused an error if expr() was called again, but it never was. The function should have been Ge, which would have caused the entire input to be consumed, since it was a valid comparator expression.

The fix is to add a program rule to the grammar which is defined as expr + EOF. This will inspire the parser to consume the entire input, raising one of several different types of errors if it can't. In this case, we would have hit an AttributeError when it tried to look up the function Gte in func.py, since the unrecognized string would be parsed as an infix expression. (We will now catch that exception and re-raise as a SyntaxError.)