- Move io, run and rand to core.
 - Remove incorrect ctypes module (use libc).
 - Remove os-specific modules for os and fs.
 - Split fs between core::path and core::os.

This mod only had two functions, all of whose users have been changed
to use the uint module.

As per discussion on IRC. I am about to file an RFC for further
discussion about the more general issue of whether to enforce
invariants through types, typestate, or dynamic checks, but for now,
removing the misleading name "last_unsafe".

See Issue 1943 for any discussion (reopen it if necessary).

Closes #1943

Instead, use vec::to_mut/from_mut to transform vectors in place as
needed.

Closes #1805

This way makes it equivalent to the compiler's vec push, and is a lot
faster than calling vec::grow.

`with_scope` and `with_cond` can be used to wrap a piece of code in a
scope block, or conditionalize it on a value, without doing all the
context-creation and jumping by hand.

Also renames @block_ctxt to block to reduce noise.

Make process-spawning take environments and working directories, remove procsrv task from compiletest.