Add some lemmas to PathGroupoids
Classify the path space of precategories

After    | File Name                 | Before   || Change
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0m06.31s | Total                     | 0m02.12s || +0m04.19s
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0m04.16s | categories/Category/Paths |   N/A    || +0m04.16s
0m01.63s | contrib/HoTTBook          | 0m01.69s || -0m00.06s
0m00.42s | categories                | 0m00.36s || +0m00.06s
0m00.10s | categories/Category       | 0m00.07s || +0m00.03s
Prove that groupoids are self-dual
Merge pull request #649 from JasonGross/path-precategory

Path space of a precategory