Fix deprecations (#59)

In the massive deletion of deprecated functions when rc1 was released (#59),
`Base.has`, `Base.ref` and `Base.assign` were removed.

I've simply find/replaced them with their replacements (`haskey`, `getindex`
and `setindex!`), so I might very well have introduced new problems here,
but all tests pass and my own debugging was able to progress smoothly (where
it previously broke as soon as I tried to print something).
This line should not be changed; `:ref` is still the head used in indexing `Expr`s.