Merge the `use` parts of the `no_std` branch. This reduces the diffs
between master and the `no_std` branch, making it easier to maintain.
Most of these changes are derived from patches by @lachlansneff in
https://github.com/Cretonne/cretonne/tree/no_std.
This will allow wasm implementations that wish to insert code into
every loop, for example to insert an interrupt check or a safepoint.
do so without relying on asynchronous signals.
