when using only libnative (or libgreen), there should be no dynamic dispatch

[thestinger]

This results in a significant amount of code bloat, forcing instruction cache churn. It also prevents a very large constant amount of dead code from being eliminated and greatly increases the compile-time with link-time optimization. At the moment, a hello world binary compiled with rustc foo.rs -Z lto -O -C link-args="-s -Wl,--as-needed,--gc-sections" is 400k. It should take at most a few kilobytes for the Rust-specific wrapper around main, leaving the binary under 10k.

[alexcrichton]

Closing as a dupe of #11560.

Implementing that architectural change would allow elimination of all unused I/O code.