The Alembic importer uses a linear search over the mesh edges to find
the right edge when setting edge creases. Although the complexity is
`O(m * n)`, with `m` being the number of creased edges, and `n` being
the number of edges, this can lead to a quadratic complexity as `m`
approches `n`.
This patch uses `EdgeHash` to store and retrieve the edges, which
should bring complexity closer to `O(n)`, provided that lookup is
`O(1)`.
See differential for some timings. In most files, this is expected
to give at least a 2-3x speedup for this operation, but can lead
orders of magnitude speed increase for dense meshes with a significant
number of edge creases.
Differential Revision: https://developer.blender.org/D15521
f7d5aaa365