unpack into renamed local variables

[tpapp]

See mauro3/UnPack.jl#18, for which I made a PR which got no response. I am wondering if you would accept a similar feature in this package.

[devmotion]

Generally, the main motivation for SimpleUnPack is to be simple and solely based on Base functionality (getproperty/getfield, and I would like to add similar convenience macros for setproperty!/setfield!). This ensures that there are no surprises and that it is consistent with the destructuring syntax in Base. So the number of features I'd like to add are quite limited.

That being said, I think your proposal sounds reasonable. IMO @unpack a as a1, b as c, ... = x would have been neat but we can't use as as the expression is not parsed correctly. So maybe a => a1 is fine.