Inline toString memoization in Key.
Inline toString memoization in Key.

This decreases the cost of toString slightly (~20.97ns->~14.31ns) and removes the
Suppliers (2 objects per key).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=115866891

cgruber/guice,cgruber/guice,takecy/guice,mcculls/guice,mcculls/guice,takecy/guice,sonatype/sisu-guice,google/guice,mcculls/guice,google/guice,google/guice,sonatype/sisu-guice