refactor: Principal overhaul

[lwshang]

This refactor introduced breaking changes, so bumped up version to 0.4.0-beta.0.

Breaking changes

• Remove PrincipalInner
  • Principal directly holds len and bytes fields
• PrincipalError enum has different set of variants reflecting changes in from_text logic.
• from_text accepts input containing uppercase letters which results in Err before.
• from_text verifies CRC32 check sequence

Implementation details

• Mark Principal::from_slice as deprecated
  • It panics if input longer than 29 bytes
  • recommend to use try_from_slice
• Move all tests to root/tests folder so that we only test public visible behavior
• Use data-encoding instead of base32
  • base32 wrongly accepts invalid base32 string which makes error handling in from_text not clear
  • data-encoding is better maintained and has more dependents and downloads
• Use serde_test
  • serde_json and serde_cbor are kept in dev-dependencies because of doc-tests
• Use impl::impl! macro to verify the necessary traits are implemented

Discussion about data layout

struct Principal(PrincipleInner);
#[repr(packed)]
struct PrincipleInner {
  len: u8,
  bytes: [u8; 29],
}

After removing the layer of PrincipalInner, it becomes:

#[repr(packed)]
struct Principal {
  len: u8,
  bytes: [u8; 29],
}

I'm not sure whether we should keep the repr(packed) here. The Rust Nomicon says:

repr(packed) forces Rust to strip any padding, and only align the type to a byte. This may improve the memory footprint, but will likely have other negative side-effects.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8b19c6d46a7600f575cc936c9dd746ca
With or without repr(packed), the type always has size of 30.

[lwshang]

I created a repo to illustrate that with or without #[repr(packed)], Principal always has size of 30 on wasm32 platform.
So it should be safe to remove the annotation.

I also made a draft PR to candid which makes sure that candid won't be broken.