Improve compile time (#1989)

## Issue Addressed

Closes #1264

## Proposed Changes

* Milagro BLS: tweak the feature flags so that Milagro doesn't get compiled if we're using BLST. Profiling showed that it was consuming about 1 minute of CPU time out of 60 minutes of CPU time (real time ~15 mins). A 1.6% saving.
* Reduce monomorphization: compiling for 3 different `EthSpec` types causes a heck of a lot of generic functions to be instantiated (monomorphized). Removing 2 of 3 cuts the LLVM+linking step from around 250 seconds to 180 seconds, a saving of 70 seconds (real time!). This applies only to `make` and not the CI build, because we test with the minimal spec on CI.
* Update `web3` crate to v0.13. This is perhaps the most controversial change, because it requires axing some deposit contract tools from `lcli`. I suspect these tools weren't used much anyway, and could be maintained separately, but I'm also happy to revert this change. However, it does save us a lot of compile time. With #1839, we now have 3 versions of Tokio (and all of Tokio's deps). This change brings us down to 2 versions, but 1 should be achievable once web3 (and reqwest) move to Tokio 0.3.
* Remove `lcli` from the Docker image. It's a dev tool and can be built from the repo if required.

crypto/bls/src/impls/mod.rs

@@ -1,3 +1,4 @@
 pub mod blst;
 pub mod fake_crypto;
+#[cfg(feature = "milagro")]
 pub mod milagro;

crypto/bls/src/lib.rs

@@ -42,6 +42,7 @@ pub use get_withdrawal_credentials::get_withdrawal_credentials;
 pub use zeroize_hash::ZeroizeHash;
 
 use blst::BLST_ERROR as BlstError;
+#[cfg(feature = "milagro")]
 use milagro_bls::AmclError;
 
 pub type Hash256 = ethereum_types::H256;
@@ -49,6 +50,7 @@ pub type Hash256 = ethereum_types::H256;
 #[derive(Clone, Debug, PartialEq)]
 pub enum Error {
     /// An error was raised from the Milagro BLS library.
+    #[cfg(feature = "milagro")]
     MilagroError(AmclError),
     /// An error was raised from the Supranational BLST BLS library.
     BlstError(BlstError),
@@ -62,6 +64,7 @@ pub enum Error {
     InvalidZeroSecretKey,
 }
 
+#[cfg(feature = "milagro")]
 impl From<AmclError> for Error {
     fn from(e: AmclError) -> Error {
         Error::MilagroError(e)
@@ -122,6 +125,7 @@ macro_rules! define_mod {
     };
 }
 
+#[cfg(feature = "milagro")]
 define_mod!(milagro_implementations, crate::impls::milagro::types);
 define_mod!(blst_implementations, crate::impls::blst::types);
 #[cfg(feature = "fake_crypto")]