Custom encoder/decoder must receive allocator

[jsign]

This PR makes the following changes:

• It makes the custom encoder and decoder receive an allocator. This is required since both encoding and decoding need an allocator thus custom ones need one too. (i.e: the same allocator, so all allocations in potentially nested structs with custom allocators be allocator-coherent).
• Move the serialize/deserialize parameters such that the allocator : Allocator is the first parameter, since this is a usual Zig idiom.

[jsign]

I did this renaming since the encoder is named encodeToRLP -- to be coherent.
We could also remove the To in the encoder and have encodeRLP and decodeRLP but I think I like most this style.

[gballet]

LGTM