This changes some type signatures, which is technically a breaking
change (although the previous implementations were essentially unusable
anyway).

This requires upgrading the dependency on `probability` from 0.17
to 0.18, which is technically a breaking change since `constriction`
re-exports `probability::distribution::Distribution` and
`probability::distribution::Inverse` unless compiled without the default
feature "std".

Starting from version 0.19, crate `probability` is now supposed to
both support `no_std` mode and remove all external syscalls for
mathematical special functions. We therefore no longer need to make the
crate `probability` an optional dependency.

The updated `probability` *should* allow users to use `constriction` in
`no_std` mode in more contexts, and to run more tests in `miri`. Whether
this is actually the case will be tested in the next few commits.

The new version of the `probability` crate no longer uses system calls
for special mathematical functions, so we can now run (almost) all tests
in miri.

The only tests that are still disabled in miri are the benchmarks in
directory `benches`. These cannot be enabled at this point since
`criterion` seems to be incompatible with `miri`.

First step towards `no_std` compatibility.

This reverts commit 63f6252.

For `no_std` compatibility, we can use `log2` only if either feature
"std" or feature "libm" is active.

This one is `no_std` compatible

The only missing piece was to replace `f64::ln_1p` with `libm::log1p`.