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)

common/lighthouse_version/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "lighthouse_version"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+git-version = "0.3.4"
+target_info = "0.1.0"

common/lighthouse_version/src/lib.rs

@@ -0,0 +1,24 @@
+use git_version::git_version;
+use target_info::Target;
+
+/// Returns the current version of this build of Lighthouse.
+///
+/// A plus-sign (`+`) is appended to the git commit if the tree is dirty.
+///
+/// ## Example
+///
+/// `Lighthouse/v0.2.0-1419501f2+`
+pub const VERSION: &str = git_version!(
+    args = ["--always", "--dirty=+"],
+    prefix = "Lighthouse/v0.2.0/",
+    fallback = "unknown"
+);
+
+/// Returns `VERSION`, but with platform information appended to the end.
+///
+/// ## Example
+///
+/// `Lighthouse/v0.2.0-1419501f2+/x86_64-linux`
+pub fn version_with_platform() -> String {
+    format!("{}/{}-{}", VERSION, Target::arch(), Target::os())
+}