- Allow wallets and other clients to parse blocks from both coreth and subnet-evm without registering extra types
- Allow wallets and other clients to make their genesis/chain configurations without registering extra types
## Why this should be merged
Consolidates duplicated logic. Similar rationale to #84.
## How this works
New `register.AtMostOnce[T]` type is responsible for limiting calls to
`Register()`.
## How this was tested
Existing unit tests of `params`. Note that the equivalent functionality
in `types` wasn't tested but now is.
## Why this should be merged
Cleans up loose ends after renaming the Go module. Also adds an
introduction to the README to explain the purpose of libevm.
## How this works
The changed hash in the workflow is just a fix (although a no-op). The
spaces after the copyright headers are to stop them from [showing up in
documentation](https://pkg.go.dev/github.com/ava-labs/libevm@v1.13.14-0.1.0-rc.1/params).
## How this was tested
n/a
* refactor: extra types `C` + `R` are never plumbed as `*C` / `*R`
* refactor: force use of `pseudo.Constructor.Zero()` instead of `NilPointer()`
* feat: `pseudo.PointerTo()`
* feat: `params.ExtraPayloadGetter[C,R].PointerFromChainConfig(...) *C` and `Rules => *R` equiv
* test: shallow copy of `ChainConfig`/`Rules` includes extras
* feat: `params.ChainConfig` extra payload can use root JSON
* refactor: simplify `ChainConfig.UnmarshalJSON()` branches
* fix: change redundant `assert` to `require` for simplicity
