Document that "at" and "get" have special meaning on steps

[benjie]

• at should accept a single number argument and implies that the step is list-like (and you're accessing that index)
• get should accept a single string argument and implies that the step is object-like (and you're accessing that key)

If steps implement these methods but don't conform to expectations, then odd things might happen.

[benjie]

See isObjectLikeStep and isListLikeStep functions (the latter is currently on a branch)

[benjie]

@jemgillam please can you add details of this convention to the Grafast docs, probably in the section talking about writing your own step classes (since these are step class methods)?