Alias Proc#[] to #call

[postmodern]

I think it would be useful to alias Proc#[] to #call.

1. Allows using procs interchangeably with Hashes. This is useful for when you wish to cache the results of a calculation when the key-space is rather small, or "call" a proc directly when the key-space is too large to be efficiently cached. In both instances the calling code would simply be proc_or_cache[key].
2. Emulates the idea that procs are mathematical functions which map input values to associated output values.
3. Provides compatibility with Ruby.

[Blacksmoke16]

I remember seeing another issue related to this, but can't seem to find it...

[nobodywasishere]

Potentially similar to #2162