Add two solutions to euler #41.

One of them (41-take2.rs) requires a still-not-accepted patch to the Rust
standard library; I'm not sure if it will ever be accepted, either.
https://github.com/mozilla/rust/pull/13761
Add euler #43
Add euler #44
Add euler #45
Add euler #46
#41: whitespace edit; #44: move is_pentagonal to exscape.rs
Add euler #42; looks like I missed adding it previously
Add euler #24.

I've waited with adding this one, since next_permutation() is not part of Rust...
At least not yet. However, since I've committed other problems using next/prev_permutation()
by now, I figured I might as well add the older ones, too.
Add is_pandigital, is_triangular, is_pentagonal, is_hexagonal to exscape.rs.

Unfortunately, I've only bothered writing tests and proper docs for one of them.
Update permutations.rs.

It's now more in line with the pull request.
All functions are renamed to not collide with the patched compiler, though.
Add euler #47