Added accent sensitive and case insensitive collation, utf8mb4_0900_as_ci.
The implementation is simple and straightforward because all the logic of
multi-level collation is ready when we added accent and case sensitive
collations.

The benchmark result of this collation is slower than accent and case
insensitive collation but faster than accent and case sensitive collation,
just as expected.

BM_MixedUTF8MB4             161 ns/iter     0.77 GB/sec
BM_MixedUTF8MB4_AS_CI       336 ns/iter   377.31 MB/sec
BM_MixedUTF8MB4_AS_CS       486 ns/iter   261.13 MB/sec

Change-Id: I1e44b7008c7746ee111dd8738d22591c7a36eec4