Less allocated objects on each request

How many? Using `memory_profiler` and a Rails app (codetriage.com), master uses:

```
rack/lib x 7318
```

After this patch, the app uses:

```
rack/lib x 4598
```

Or `(7318 - 4598) / 7318.0 * 100 # => 37.16` % fewer objects __PER REQUEST__.

To do this, I extracted really commonly used strings into top level Rack constants. It makes for a bit of a big diff, but I believe the changes are worth it. 

Running benchmark/ips against the same app, I'm seeing a performance boost of `2~4%` across the entire app response. This doesn't just make Rack faster, it will make your app faster.

While we could certainly go overboard and pre-define ALL strings as constants, that would be pretty gnarly to work with. This patch goes after the largest of the low hanging fruit.
My favorite type of comment on an open source pull request: one that teaches me something i didn't know. You are correct. I tested this out with memory_profiler and funny enough, it doesn't even show a single object allocated


```
    report = MemoryProfiler.report do
      num.times {
        foo = /^to_ary$/
       }
    end
    report.pretty_print

Total allocated 0
Total retained 0

allocated memory by gem
-----------------------------------

allocated memory by file
-----------------------------------

allocated memory by location
-----------------------------------

allocated objects by gem
-----------------------------------

allocated objects by file
-----------------------------------

allocated objects by location
-----------------------------------

retained memory by gem
-----------------------------------

retained memory by file
-----------------------------------

retained memory by location
-----------------------------------

retained objects by gem
-----------------------------------

retained objects by file
-----------------------------------

retained objects by location
-----------------------------------


Allocated String Report
-----------------------------------

Retained String Report
-----------------------------------
```