partial_sort contains Out-of-bounds Read in release mode

Summary

Affected versions of this crate were using a debug assertion to validate the `last` parameter of `partial_sort()`. This would allow invalid inputs to cause an out-of-bounds read instead of immediately panicking, when compiled without debug assertions. All writes are bounds-checked, so the out-of-bounds memory access is read-only. This also means that the first attempted out-of-bounds write will panic, limiting the possible reads. The accessible region is further limited by an initial bounds-checked read at `(last / 2) - 1`, i.e., it is proportional to the size of the vector. This bug has been fixed in v0.2.0.