TimeEvolution with buffer

[termoshtt]

I drop "self-consuming" style API, which was originally introduced in order to accept KSE code in rust-spectral.
It is hard to use same interface among &self and &mut self time-evolution operator.

This PR introduces Buffer type which can be mutable while the EOM itself immutable.

[termoshtt]

master

cargo run --release --bin bench  16.15s user 0.21s system 100% cpu 16.200 total

buffered

cargo run --release --bin bench  9.30s user 0.24s system 101% cpu 9.383 total

almost twice faster (/・ω・)/

[termoshtt]

cargo run --release --bin bench  8.15s user 0.22s system 102% cpu 8.193 total

[termoshtt]

Constructor of RK4Buffer may be more simple like

let buf = teo.new_buffer();

But it needs breaking change.
It postponed another PR.