Merge #182

182: Add initial Cluster support r=jamesmunns a=jamesmunns

Add cluster support to svd2rust. I am calling it initial, because I have only tested it against the `nRF52` [svd file](https://github.com/jamesmunns/nrf52/blob/master/nrf52.svd).

Note, this is based on #180, so I would prefer that PR be merged first. If that PR is rejected, I can instead rebase these changes on `japaric/master` (just let me know).

Edit: All changes relevant to clusters are squashed into the last commit of this PR, so if you want to see the relevant changes, check out 6de6de7.

This is based on the work of @brandonedens earlier #149, and I believe would close #107 and related issues.

CC @japaric @Emilgardis @therealprof

Edit 2: Here are some handy references:
* [SVD snippet with clusters](https://gist.github.com/jamesmunns/c4e53fe5bd74dca81fdbff6bb1798ddd)
* [code generated before](https://gist.github.com/jamesmunns/d854a6c2665cca59edb88143d82a19c6)
* [code generated after](https://gist.github.com/jamesmunns/7558667e34c33124c60b3aaaf679a196)
* [diff of code generated](https://gist.github.com/jamesmunns/03fd0d7d3595cd0816dbfa8daca0a553)

Author: bors[bot]

Cargo.toml

@@ -29,8 +29,9 @@ path = "src/main.rs"
 [dependencies]
 cast = "0.2.2"
 clap = "2.26.0"
+either = "1.0.3"
 error-chain = "0.11.0"
 inflections = "1.1.0"
 quote = "0.3.15"
-svd-parser = "0.5.2"
+svd-parser = "0.6"
 syn = "0.11.11"

src/generate/device.rs

@@ -124,6 +124,7 @@ pub fn render(d: &Device, target: &Target) -> Result<Vec<Tokens>> {
             continue;
         }
 
+
         out.extend(peripheral::render(p, &d.peripherals, &d.defaults)?);
 
         if p.registers