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See #349

Changes:

• #274 – on-disk build
• Fix for xcode: #334
• Minor fix when saving an index to the same file as a current index: #335

• Fixes to the Euclidean distance function (avoid catastrophic cancellation)
• Don't MAP_POPULATE by default

Main (only?) change is that dot products are now supported thanks to @psobot!

Finally squashed a remaining issue with holes (see #295) and together with a few other holes-related fixes it felt worthy of bumping the minor version number.

Fixes #279 and #261

Includes #276

This version features AVX instructions thanks to @ReneHollander

Two big updates

• Windows support now official – with CI pipeline to prove it. Only tested on Python 3.6 so far
• Experimental Hamming distances

• Fixes a problem with holes in the index reported by @yonromai in #223