Lighthouse v0.2.0 (Medalla) (#1452)

## Issue Addressed

NA

## Proposed Changes

- Moves the git-based versioning we were doing into the `lighthouse_version` crate in `common`.
- Removes the `beacon_node/version` crate, replacing it with `lighthouse_version`.
- Bumps the version to `v0.2.0`.

## Additional Info

There are now two types of version string:

1. `const VERSION: &str = Lighthouse/v0.2.0-1419501f2+`
1. `version_with_platform() = Lighthouse/v0.2.0-1419501f2+/x86_64-linux`

(1) is handy cause it's a `const` and shorter. (2) has platform info so it's more useful. Note that the plus-sign (`+`) indicates the the git commit is dirty (it used to be `(modified)` but I had to shorten it to fit into graffiti).

These version strings are now included on:

- `lighthouse --version`
- `lcli --version`
- `curl localhost:5052/node/version`
- p2p messages when we communicate our version

You can update the version by changing this constant (version is not related to a `Cargo.toml`):

b9ad7102d5/common/lighthouse_version/src/lib.rs (L4-L15)

lcli/Cargo.toml

@@ -34,3 +34,4 @@ eth2_libp2p = { path = "../beacon_node/eth2_libp2p" }
 validator_dir = { path = "../common/validator_dir", features = ["insecure_keys"] }
 rand = "0.7.2"
 eth2_keystore = { path = "../crypto/eth2_keystore" }
+lighthouse_version = { path = "../common/lighthouse_version" }

lcli/src/main.rs

@@ -1,6 +1,5 @@
 #[macro_use]
 extern crate log;
-
 mod change_genesis_time;
 mod check_deposit_data;
 mod deploy_deposit_contract;
@@ -29,6 +28,7 @@ fn main() {
     simple_logger::init_with_level(Level::Info).expect("logger should initialize");
 
     let matches = App::new("Lighthouse CLI Tool")
+        .version(lighthouse_version::VERSION)
         .about(
             "Performs various testing-related tasks, including defining testnets.",
         )