Elaborate on JIT, code unloading, fragmentation

[jfbastien]

Opening a bug to write down concrete wording from discussion in:
WebAssembly/spec#32

[sunfishcode]

I believe the intend link is to #32.

Clearing the "bug" label (but leaving this issue open of course) since this is about ideas described in FutureFeatures.md not being fully fleshed out, which is not unexpected in FutureFeatures.md.

[sunfishcode]

This was fixed in #125.