Reduce memory consumption in wavelet matrix builder

[kampersanda]

This PR uses CompactVector instead of Vec for buffers in WaveletMatrixBuilder, allowing us to specify the minimum number of bits to store an input integer for working memory efficiency.

I measured the peak resident set size during constructing WaveletMatrix from $2^{26}$ random bytes (code is here).

# v0.5.0
$ /usr/bin/time ../target/release/wmb
[bench/src/wmb.rs:16] wm.len() = 67108864
4.09user 0.41system 0:04.52elapsed 99%CPU (0avgtext+0avgdata 1152920maxresident)k
0inputs+0outputs (0major+35199minor)pagefaults 0swaps

# New version (will be v0.6.0)
$ /usr/bin/time ../target/release/wmb
[bench/src/wmb.rs:17] wm.len() = 67108864
5.83user 0.10system 0:05.93elapsed 99%CPU (0avgtext+0avgdata 244280maxresident)k
0inputs+0outputs (0major+68665minor)pagefaults 0swaps

The working memory was reduced from 1.1 GB to 0.24 GB, although the speed was slower.

In this PR, I removed the function WaveletMatrix::from_ints() because this can lead to a memory inefficient construction way.

[hirosassa]

Cool PR! Looks good.

I removed the function WaveletMatrix::from_ints() because this can lead to a memory inefficient construction way.

I agree with your idea. Your newer implementation provides simple API for users.