`std` features not included by default

[sebastianrex]

It looks like the 0.30 release changed the default features, which is reasonable.

However, looking at https://github.com/libpnet/libpnet/blob/master/Cargo.toml#L22-L23, my reading is that the desire was to include std by default, since default includes the defaults from all the sub-crates, and those all include std by default.

However, the base std feature isn't included by default, which means that ipnetwork isn't included by default either.

This surprised me, as I had to add:

pnet = { version = "0.31", features = ["default", "std"]}

to my Cargo.toml to get the std feature enabled, in order to use ipnetwork via pnet.

I haven't submitted a PR because I wasn't sure if this was intentional or not, but it looked accidental.

[arctic-alpaca]

This also greatly reduces the documented code on docs.rs compared to version 0.30.

[mrmonday]

This should be fixed from v0.32.0.