remove allocator API requirement

[glommer]

trafficstars

I figured I'd have to adapt the code a bit to remove that, but right now as of 2020 just by removing the feature the code compile just fine with the stable compiler:

My environment: $ rustup default stable-x86_64-unknown-linux-gnu (default)

I intend to use this as a building block for my crate, Scipio (https://crates.io/crates/scipio) but I'd like to enforce stable there.

[spearman]

I believe this is a duplicate of #22