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)

beacon_node/rest_api/tests/test.rs

@@ -24,7 +24,6 @@ use types::{
     RelativeEpoch, Signature, SignedAggregateAndProof, SignedBeaconBlock, SignedRoot, Slot,
     SubnetId, Validator,
 };
-use version;
 
 type E = MinimalEthSpec;
 
@@ -764,7 +763,11 @@ fn get_version() {
         .block_on(remote_node.http.node().get_version())
         .expect("should fetch eth2 config from http api");
 
-    assert_eq!(version::version(), version, "result should be as expected");
+    assert_eq!(
+        lighthouse_version::version_with_platform(),
+        version,
+        "result should be as expected"
+    );
 }
 
 #[test]