This is especially useful for the native calls in std::math

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Not included in the build by default, since it's fragile and kludgy. Do
something like this to run it:

    cd doc/tutorial
    RUSTC=../../build/stage2/bin/rustc bash test.sh

Closes #1143

Closes #989

Closes #1209

Closes #1117

Closes #1153

Also simplifies the literal-munging, and moves it into ast_util

Closes #1170

If visit_ty is not overridden, it uses a stub function which does not
descend into types.

Closes #1204

This is useful for writing caches and perhaps in typestate predicates.
It also adds a companion module for bool with from_str, to_str, ... fns.

It fails under valgrind

Thanks to marijn for helping with #[link_name]

The safe-reference checker requires a copy of each mapped-over element
only when the vector is mutable. Let's not pay that cost for immutable
vectors.

Closes #1188

The last-use pass now takes input from the alias pass to not mark things
as last uses that are still accessed through a reference.

Issue #925

This should allow a small speedup for test builds; unset by default

These uses aren't really doing a full yield. They are just giving up control
to the scheduler and will never return.

This intrinsic existed just to get ahold of the return pointer. I replaced it
with a call_with_retptr intrinsic that grabs the return pointer and passes it
to another Rust function, thereby eliminating the need to call C functions on
the Rust stack.