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

Need this for building istrs

This leaves syntax::ast just defining the AST, which strikes me as somewhat
nicer

The syntax is

    alt x {
        mypat where mycond { ... }
    }

The condition may refer to any of the variables bound by the pattern.
When a guard fails, pattern-matching continues with the next pattern.

Closes #857

This is not currently necessary, but will be, when I land the new ivec
representation.

(I'll be using this for type-needs-copy-glue in the near future.)

Since they are now passed by pointer, this is no longer a problem