Give Join a way to alias the joining table. Fixes #1007 (#1013) #1074

Job	Run time
check_format	1m 39s
specs (shard.yml, 12, 1.6.2, false)	4m 34s
specs (shard.yml, 12, latest, false)	3m 35s
specs (shard.yml, 13, 1.6.2, false)	4m 30s
specs (shard.yml, 13, latest, false)	3m 45s
specs (shard.yml, 14, 1.6.2, false)	4m 15s
specs (shard.yml, 14, latest, false)	4m 16s
specs (shard.edge.yml, latest, 14, true)	3m 52s
specs (shard.yml, nightly, 14, true)	3m 31s
	33m 57s

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