Fix no_std builds attempt 2 (#78)

* Fix no_std builds attempt 2

- Turn off default features for core in gen and genmult
- Use serde-json-core (no_std support) in dev-dependencies

Relates to #71

* Use resolver="2"

Allows us to use serde_json in dev-dependencies and compile for no_std:

https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#cargos-new-feature-resolver

* revert unnecessary changes

Cargo.toml

@@ -7,6 +7,7 @@ authors = ["Wei Tang <hi@that.world>"]
 repository = "https://github.com/paritytech/libsecp256k1"
 keywords = ["crypto", "ECDSA", "secp256k1", "bitcoin", "no_std"]
 edition = "2018"
+resolver = "2"
 
 [dependencies]
 libsecp256k1-core = { version = "0.2.2", path = "core", default-features = false }