Sync in the latest CloudABI generated source files.

Languages like C++17 and Go provide direct support for slice types:
pointer/length pairs. The CloudABI generator now has more complete for
this, meaning that for the C binding, pointer/length pairs now use an
automatic naming scheme of ${name} and ${name}_len.

Apart from this change and some reformatting, the ABI definitions are
identical. Binary compatibility is preserved entirely.