Resolve lambdas in Container __getattr__

[ev1313]

Currently using Rebuild is only useful when parsing, because the dictionary gets filled with value copys:

    d = Struct(
        "foo" / Int32ul,
        "bar" / Rebuild(Int32ul, lambda ctx: ctx.baz),
        "baz" / Rebuild(Int32ul, lambda ctx: ctx.foo),
    )
    data = b'\x04\x00\x00\x00\x04\x00\x00\x00\x04\x00\x00\x00'
    obj = d.parse(data)
    # Container(foo=4, bar=4, baz=4)

However this isn't useful when building, because you don't know in later stages about the rebuilding, and when the parameters change, you'd need to change all of them.

With this change it is possible to put lambda ctx: ctx.foo into the Container, which get resolved immediately, when accessed. (See the test case)

This could be expanded upon, so that Rebuild itself just add these to the Container when parsing, or that a possible preprocessing step adds these before building.

The inspect hack is necessary to not break the existing behaviour of Lazy adding lambdas into the Container, which get resolved and return not the lambda, but the resulting value directly.

[JayFoxRox]

Why isn't this just {"foo": 4}? Asked differently: Why are "bar" and "baz" functions?

I think it's a bit hacky to allow the data to decide how it should be processed (~ functions). I'm not sure if there's a precedent for this in construct.

Instead, the lambda ctx: ctx.baz in Rebuild should be deferred until ctx.baz is available.

Is there an actual benefit doing it the proposed way (allowing functions in input to build) over deferring Rebuild?