Add pop_n function

[knsd]

Hi, this pr adds pop_n function for the same reasons as in #8, I think it could be useful, this function attempts to pop at most target.len() values off the buffer.

Benchmarks:

pop_n_via_pop           time:   [737.92 ns 741.77 ns 746.32 ns]
pop_n                   time:   [79.724 ns 80.440 ns 81.353 ns]