This PR adjusts the experimental module system to make `private` the
default visibility modifier in `module`s, introducing `public` as a new
modifier instead. `public section` can be used to revert the default for
an entire section, though this is more intended to ease gradual adoption
of the new semantics such as in `Init` (and soon `Std`) where they
should be replaced by a future decl-by-decl re-review of visibilities.
This commit fixes issue reported at
https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/deterministic.20timeout/near/287654818
Type class resolution was diverging when trying to synthesize
```lean
HSub (optParam Float 0.0) (optParam Float 1.0) ?m.472
```
and Lean was diverging while unfolding
```lean
instance : OfScientific Float where
ofScientific m s e :=
if s then
let s := 64 - m.log2 -- ensure we have 64 bits of mantissa left after division
let m := (m <<< (3 * e + s)) / 5^e
Float.ofBinaryScientific m (-4 * e - s)
else
Float.ofBinaryScientific (m * 5^e) e
```
was being unfolded.
Anothe potential solution for the problem above is to erase the
`optParam` annotations before performing type class resolution.
