Fixup namespace for publics.

[MikeStall]

Apply renames to publics. See PublicSurfaceTest for principles:

        // The goal for public namespaces is to make the SDK easy for the consumer. 
        // Namespace principles for public classes:            // 
        // - prefer fewer namespaces. See C# for example: https://docs.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis
        // - For easy discovery, but Engine in "Microsoft.PowerFx".
        // - For sub areas with many related classes, cluster into a single subnamespace.
        // - Avoid nesting more than 1 level deep

This tries to just change using and namespaces - just update public surface area and minimize churn.

[coveralls]

Pull Request Test Coverage Report for Build 302566

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3342 unchanged lines in 128 files lost coverage.
• Overall coverage remained the same at 57.358%

---

Files with Coverage Reduction	New Missed Lines	%
/src/libraries/Microsoft.PowerFx.Core/Public/Engine.cs	1	98.92%
/src/libraries/Microsoft.PowerFx.Core/Public/Types/AggregateType.cs	1	95.0%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/BooleanValue.cs	1	57.14%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/DateTimeValue.cs	1	62.5%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/DateValue.cs	1	66.67%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/FormulaValueNewRecord.cs	1	82.35%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/FormulaValueNewTable.cs	1	90.91%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/GuidValue.cs	1	57.14%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/NumberValue.cs	1	57.14%
/src/libraries/Microsoft.PowerFx.Core/Public/Values/StringValue.cs	1	70.0%

Totals
Change from base Build 302540:	0.0%
Covered Lines:	19382
Relevant Lines:	33791

---

💛 - Coveralls