Macro usage in the library

[FreezyLemon]

From a Github issue:

also wondering why the parser was built using so many macros. seems hard to understand/maintain.

I understand this point. Reading and parsing some of the macros in the crate is difficult. On the other hand, the macros reduce code repetition by a lot (try cargo-expand and grep for EbmlParsable to see what I mean).

[FreezyLemon]

Moved over from hackmd, original comment by @Luni-4

Perhaps we can try to reduce macros whenever it's possible, for example when they are difficult to read, and use them when there are no other alternatives.