Use array of concrete eltype for efficiency in astar

[jlapeyre]

An array of eltype Integer is used in the astar algorithm. But, the elements are always only of a single concrete subtype of Integer. This PR changes the array to the narrower type. Timing the second test of astar in the test suite (applying astar to the complete graph) shows a 2x improvement in speed.

EDIT: it would be better if ones(Integer, n) threw an error, rather than arbitrarily choosing to fill with Ints. But, that ship has sailed.

[codecov]

Codecov Report

Merging #1585 (2583f80) into master (712b8d1) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 2583f80 differs from pull request most recent head 3c32020. Consider uploading reports for the commit 3c32020 to get more accurate results

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