Remove expression cache

@pushrax & @jasonhl for review

## Problem

Pull request #391 separated `Context#[]` which parsed and rendered expressions into `Expression.parse` and  `context.evaluate` to avoid memoize expression parsing.  However, parsing was still happening during rendering, which prevents us from caching the parsed expression along with the parsed template in memcache.

## Solution

Use `Expression.parse` and `Context#evaluate` in all the tags instead of `Context#[]`, then remove the expression cache, since we each expression will only be cached once.

## Benchmark

Here is the benchmark this branch:

```
              parse:       18.3 (±10.9%) i/s -       1078 in  60.034122s
        parse & run:       11.1 (±9.0%) i/s -        659 in  60.040146s
```

Here is the benchmark on master:

```
              parse:       20.1 (±14.9%) i/s -       1185 in  59.993718s
        parse & run:       11.1 (±9.0%) i/s -        662 in  60.003203s
```

## Future Work

The strict parser has been parsing expressions then converting them back into strings to use with `Context#[]`.  This refactoring will make it easier to just parse the expression once in the strict parser.
An include tag takes an expression, even though it is normally just a string.  This evaluates the expression to show what the actual template rather that the expression string.

E.g. if they had `{% include 'product' %}` then Liquid::Profiler::Timing#partial would return `"'product'"` just as it did before.  However, if they had `{% include snippet_name %}` and the snippet_name variable was assigned the value `'product'`, then previously Liquid::Profiler::Timing#partial would return `"snippet_name"`, and now it will return `"'product'"` which better reflects what partial this actually is for.

I thought this was more appropriate than storing the unparsed expression for the template name just so the behaviour doesn't change.

I guess this probably doesn't need the quotes now.  It could probably just be `Profiler.profile_children(context.evaluate(@template_name).to_s)`

@jasonroelofs does this make sense?