Fuzzer: Respect the noInvokes flag when adding interposing invokes

[kripken]

noInvokes already stopped us from emitting new invokes. We also emit
calls to invokes, basically adding more invoke executions, by "interposing"
on exports - sticking a call in an export, where the call is to an invoke.
Respect the flag in that case too: if the user doesn't want the code growth
of invokes, they don't want either new invokes or new calls to existing
invokes.

Diff without whitespace is small (almost all comments).

cc @Liedtke