Allow jmespath to be const qualified

[nnathan]

I would like to see the ability to allow const reference to a jmespath_expression, to ensure that it's read-only and allow the compiler to do any optimisations.

It appears that the jmespath_expression::evaluate() are not const methods and therefore preventing making a useful const reference.

I was a bit concerned that the jmespath_expression was mutating itself each evaluate, but after applying a const to the evaluate() methods, and then made the jmespath_expression a const reference, it didn't seem to have any issue.

[danielaparker]

You're right, evaluate() should be const, it never mutates the state of the jmespath_expression. Fixed on master.

[nnathan]

Much appreciated, thanks for the quick turn around. I'll leave it up to you if you want to close the request.