Add type information to RexLiteral#digest, ensure planner uses just RelNode#digest for identity

[vlsi]

No description provided.

[vlsi]

Unfortunately, it adds types to each and every value for Values.tuple:

I wonder if we can add special "digest without type" for Values puposes (it can print types just once)

planBefore expected:<...ion=[>(+($0, $1), 50[)])
    LogicalValues(tuples=[[{ 10, 1 }, { 30, 3] }]])
> but was:<...ion=[>(+($0, $1), 50[:INTEGER NOT NULL)])
    LogicalValues(tuples=[[{ 10:INTEGER NOT NULL, 1:INTEGER NOT NULL }, { 30:INTEGER NOT NULL, 3:INTEGER NOT NULL] }]])
>

On the other hand, null expressions become easier to understand:

Expected: "AND(null, IS NULL(?0.a))"
     but: was "AND(null:BOOLEAN, IS NULL(?0.a))"

[vlsi]

@julianhyde , I don't quite get new SqlCollation thing here, however it looks like new here makes no difference. It might be SqlCollation#strength should be updated as well, however it does not seem to be supported in Calcite yet.

This change was required so SqlCollation.IMPLICIT singleton is propagated across all expressions.