Element serialisation

[dhardy]

Currently read and write operations in traits require an extra level of
buffering. Alternative methods for direct reading/writing could be required,
but this has some caveats: the writer must know in advance exactly how much
data will be written and must read exactly the amount of data given, failing if
data cannot be read.

A compromise would be to allow the user to choose which to implement. But this
would require a switch describing which methods should be used, which is where
associated constants come in (Rust issue #29646).

Traits this is applicable to:

Element in element.rs

• Code tag #0025

[dhardy]

Alternative: use Serde for element (de)serialisation.

Element trait would then be a bunch of super-trait requirements plus a sum default/optimised function.