Add support for DefaultSignatures extension

[phischu]

Hi, this is my attempt to add support for the DefaultSignatures extension. Probably someon should add a link in #19.

[pjonsson]

I looked at the code and that looks reasonable. The contributing file mentions that new tests should be added in a separate commit.

With that said I'm still not qualified to comment on the general design, so someone else needs to merge this.

[UnkindPartition]

Merged, thanks!

@pjonsson thanks for reviewing!

[UnkindPartition]

This test no longer passes. I'm looking into it.

[pjonsson]

I haven't run this so I don't know what is breaking, but if it's Names here's an idea:

I changed some of the source points to properly represent parenthesis and keep the underlying variable positions in 2970107. This could mean that there are fewer sourceInfoPoints stored in the tree now compared to when the patch was constructed.

[UnkindPartition]

It looks like 562ebd0 broke it. I guess I should just rebase the test.

[UnkindPartition]

Should be fixed by fcdaf2f