Fuzz frequency tuning (#2806)

We had some ad-hoc tuning of which nodes to emit more
frequently in the fuzzer, but it wasn't very good. Things like
loads and stores for example were far too rare. Also it wasn't
easy to adjust the frequencies.

This adds a simple way to adjust them, by passing a size_t
which is the "weight" of that node. Then it just makes that
number of copies of it, making it more likely to be picked.

Example output comparison:

node             before    after
================================
binary           281       365
block            898       649
break            278       144
call             182       290
call_indirect    9         42
const            808       854
drop             43        92
global.get       440       398
global.set       223       171
if               335       254
load             22        84
local.get        429       301
local.set        434       211
loop             176       99
nop              117       54
return           264       197
select           8         33
store            1         39
unary            405       304
unreachable      1         2

Lots of noise here obviously, but there are large increases
for loads and stores compared to before.

Also add a testcase of random data of the typical size the
fuzzer runs, and print metrics on it. This might help us get
a feel for how future tuning changes affect frequencies.

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