Use begin block and allow variable export

[jkrumbiegel]

Breaking: The @chain macro now creates a begin block, not a let block.
This means that variables that are assigned within the macro are available outside.
Technically, situations are imaginable where this could lead to overwritten variables if someone used large expressions with intermediate variable names in begin blocks spliced into the chain.
It is however quite unlikely for the normal way that @chain is intended to be used.

Additionally, it is now possible to use the syntax variable = some_expression to make use of the feature that variables can be exported.
The some_expression part is handled exactly like before.
This enables you to carry parts of a computation forward to a later step in the chain or outside of it:

@chain df begin
    transform(...)
    select(...)
    intermediate = subset(...)
    groupby(...)
    combine(...)
    join(intermediate)
end

@show intermediate