[code-reflection] RFR: [hat] DeviceType interface definition for private/local data structures

[Juan Fumero]

This PR introduces the `DeviceType` to be able to represent user data types in Local and Private memory of the GPU for both CUDA and OpenCL. 

As a PoC, it extends the support for FP16 (`half`) to be able to operate, load and store F16 types, even in data structures defined by the user using F16. 

Example:


    interface DevicePrivateArray2 extends DeviceType {
        F16 array(int index);                  // F16 is a predefine HAT type
        void array(int index, F16 value);

        DeviceSchema<DevicePrivateArray2> schema = DeviceSchema.of(DevicePrivateArray2.class,
                builder -> builder.withArray("array", 1024).          // build an array of this type for 1024 elemtns
                        .withDeps(F16.class, half -> half.withField("value")));   // compose of this type

        // compatible with the prev. IFace version
        static DevicePrivateArray2 create(Accelerator accelerator) {  return null; }

        // marker to create this object in private memory
        static DevicePrivateArray2 createPrivate() { return null;  }
    }


How to test? 


HAT=SHOW_CODE java -cp hat/job.jar hat.java test ffi-opencl hat.test.TestF16Type

-------------

Commit messages:
 - [hat] Fast mxm in F16 added as unittest
 - [hat] refactoring F16 name
 - [hat] refactor devicetype codegen
 - [hat] debug message cleanup
 - [hat] Test unsupported removed due to errors in CUDA
 - Merge branch 'code-reflection' into hat/device/types
 - test class reorganized
 - [hat] CUDA Backend using the new F16 builtin
 - [hat] F16 built-in type for HAT
 - [hat] DeviceType load phase refined
 - ... and 9 more: https://git.openjdk.org/babylon/compare/a1b01376...37b8417a

Changes: https://git.openjdk.org/babylon/pull/678/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=678&range=00
  Stats: 1696 lines in 38 files changed: 1320 ins; 166 del; 210 mod
  Patch: https://git.openjdk.org/babylon/pull/678.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/678/head:pull/678

PR: https://git.openjdk.org/babylon/pull/678