JMESPath let expression with operator in variable binding

[BartelNieuwenhuyse]

Fix for issue #690

JMESPath expressions with an operator in the expression part of a variable binding (expression between 'let $var=' and 'in') are not parsed correctly.
given:
{"foo": false, "bar": true}
search expression:
let $var = foo || bar in $var
expected:
yields result true
actual:
gives error during evaluation: assertion 'stack.size() == 1' failed

I've added an implicit lparen '(' when encountering the '=' of the let expression, such that the operator stack can be rewind when hitting the ',' or 'in' of the let-expression.

I've stored the name of the variable in a variable token when encountering the '=', rather than storing it in the expression_context. And use this variable token to mark the start of the binding expression, rather than storing the end_index in the expression_context. I'm not sure if after this change, expression_context has any other use as it seems to be an empty class now.

When hitting the ',' or 'in' of the let-expression the variable token and the tokens of the expression, including the operator stack up until the implicit lparen '(' are moved into the variable_expression, which on evaluation will evaluate the tokens, store the result in the context and return the original document at the start of the let expression.

Moved this logic to push_token, rather than (duplicating it) in the compile function

[danielaparker]

I figured this was coming :-) I'll review it when I have a chance. But I think the approach is basically right.