core: add support for proc static variables by reference

[msteveb]

This is an experimental feature that allows for lexical closures in Jim Tcl by allowing procs (and lambdas) to capture variables by reference rather than by value. e.g.

set a 5

proc b {} {&a} {
    incr a
}

b

Now a is 6 because b captured a by reference instead of by value