Move the experimental frontends into a separate crate, so that when not using them they don't take several minutes to compile (and indirect dependencies). (#168)

This saves several minutes (and MBs of data) on compilation time both
when running tests in this repo, but also when using the sonobe lib as a
dependency in external repos.

solidity-verifiers/Cargo.toml

@@ -29,6 +29,7 @@ ark-bn254 = {version="0.4.0", features=["r1cs"]}
 ark-grumpkin = {version="0.4.0", features=["r1cs"]}
 rand = "0.8.5"
 folding-schemes = { path = "../folding-schemes/", features=["light-test"]}
+frontends = { path = "../frontends/"}
 noname = { git = "https://github.com/dmpierre/noname" }
 
 [features]