Adding this expressions

[tomerfiliba]

I backported the this expression that I introduced to construct3 (which I don't have time to finish). So I decided it would be a good idea to backport it. It's 100% backwards compatible, so there's no harm whatsoever, and it's much more readable and concise than those ugly lambdas.

In short:

• instead of Field(..., lambda ctx: ctx.length) use Field(..., this.length)
• Instead of Field(..., lambda ctx: ctx._.length * 4) use Field(..., this._.length * 4)
• Instead of If(lambda ctx: ctx["foo"] > 17, ...), use If(this.foo > 17, ...)

I included a unit test (tests/test_this.py).

By the way, I would suggest moving the tests directory out of the package, e.g.

construct_repo/
    construct/
        core.py
        ...
    docs/
    tests/  <==