Internal variables for rules

[tomaskender]

Motivation: We want to reuse the value of an evaluated expression multiple times in a rule. This is useful not only for faster evaluation, but also for long definitions that would otherwise have to be copypasted multiple times in conditions and make them longer than they need to be.

Each rule has it's own internal variable scope.
External variables have preference over internal variables (a warning is thrown in such case).

Supported variable types: in theory everything and anything YR_OBJECT supports.
What's implemented: strings, ints, floats, bools

Syntax:

rule abc {
variables:
result= 1+1-1+1

condition:
result > 1 and result < 3
}