Organize serialization

[adria0]

As described in #176, this PR:

• Removes SerdeObject
• Renames serde.rs to encoding.rs
• Documents how fields can be serialized/deserialized (also fixes Documentation failing for 0.7.0 release #175)

[guorong009]

@adria0
I think the current update is good.
But, I think we need more updates, if we aim to organize serialization in this PR.

From this comment, I think we can remove the SerdeObject from this repository.
But, it should be done in the basis of implementing the from/to_mont for curve/field.

Please add more implementations if needed.

cc @davidnevadoc

[adria0]

From this comment, I think we can remove the SerdeObject from this repository. But, it should be done in the basis of implementing the from/to_mont for curve/field.

from/to_mont is already implemented for fields. Do you mean implementing them for curves? I do not see the specific use case the implement from/to_mont for curves (as also, AFAIS does not exist the concept of montgomery for curves, only for fields)

[adria0]

Superseded by #179