Restructure the binary_sv2 crate.

[Shourya742]

The current structure of binary_sv2 does not follow best practices. Typically, a crate defining a proc_macro only includes an additional crate for the macro’s implementation. However, in binary_sv2, we have two separate crates—one for the proc_macro implementation and another for the primitives required for macro scaffolding. Due to the tightly coupled nature of the proc_macro implementation with the module structure, restructuring it requires rewriting some parts of the macro to align with the proposed design.

Kudos to @plebhash for pointing this out and suggesting a better structure: #1459 (comment)