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/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "beacon_node"
 version = "0.1.2"
-authors = ["Paul Hauner <paul@paulhauner.com>", "Age Manning <Age@AgeManning.com"]
+authors = ["Paul Hauner <paul@paulhauner.com>", "Sigma Prime <contact@sigmaprime.io>"]
 edition = "2018"
 
 [lib]
@@ -20,7 +20,6 @@ beacon_chain = { path = "beacon_chain" }
 types = { path = "../consensus/types" }
 store = { path = "./store" }
 client = { path = "client" }
-version = { path = "version" }
 clap = "2.33.0"
 rand = "0.7.3"
 slog = { version = "2.5.2", features = ["max_level_trace", "release_max_level_trace"] }
@@ -40,3 +39,4 @@ eth2_ssz = "0.1.2"
 serde = "1.0.110"
 clap_utils = { path = "../common/clap_utils" }
 hyper = "0.13.5"
+lighthouse_version = { path = "../common/lighthouse_version" }