Merge: Performance update on Buffer

This PR is essentially the same as #1632, which has been invalidated by some black magic.

This PR heavily improves the performance on FlatBuffer by introducing cache mechanisms on indexed access.

This is possible through removal of byte-level manipulation on Buffers and re-introduction of a constant-time length attribute on Buffer.

nitmd is now practicable again and should no longer take too much time when running benches.

This PR still depends on #1628, and as such will not pass no tests as long as it is not integrated

Pull-Request: #1644

diff --cc lib/standard/text/abstract_text.nit
Simple merge