rt: Null check in walk_obj_contents. Prevents marking from crashing when calling object constructors.

rt: Factor out type param construction into from_tydesc and from_obj_shape. (The latter is unimplemented at the moment.)

This is part 1 of changing the "pred" keyword to "pure fn".
Right now, the compiler accepts both "pred" and "pure fn".

This reverts commit 629ee94a.

This wasn't a good idea after all.

This seems to be faster than creating separate ones for each block
context.

You now do

    bld::Ret(bcx, someval)

where you used to say

    bcx.build.Ret(someval)

Two fewer boxes are allocated for each block context, and build calls
no longer go through a vtable.

This is a preparation for making vectors always-on-the-heap again,
which would cause way too much malloc traffic for this idiom. I will
add an efficient std::vec::push in the future, and migrate += [x] to
that instead.

Reduces compiler code size by 3%

Closes #861

Add kind-checking for assign-op, copy, ret, be, fail exprs. Fix caught kinding-violations in rustc and libstd.

This file has suffered a lot of reformats and was looking pretty ragged

I want to use this for std::istr