Change Rust tiny-secp256k1 lib and remove wasm-pack

[fanatid]

WIP... opened for earlier feedback/discussion. Related with #9.

Proposed changes:

• Move Rust lib to the root of the repo.
• Remove secp256k1 crate and use secp256k1-sys directly. This also should resolve secp256k1::Message::from_slice fails on all-0x00 hash #7.
• Add a wrapper to Rust lib (src/lib.rs, src/wrapper.rs is secp256k1-sys wrapper). This will allow use same code for WASM and Node.js bindings (if we will decide to implement them).
• Add WASM bindings to internal Rust lib.
• Remove wasm-pack, use wasm-bindgen and wasm-opt from binaryen directly.

Possible next steps:

• Add benchmark folder with benchmarks.
• Add tests for Rust lib / WASM JS (already added?).
• Add code coverage.
• Research secp256k1-sys crate overhead compares to raw ffi.
• Research WASM size by blocklist/allowlist.
• Add Node.js / webpack usage examples.
• Add CI.
• Add documentation (rust-doc vs docs generated for ts definitions?).
• Attempt to write WASM code without std.

[fanatid]

Moved to bitcoinjs/tiny-secp256k1#53