[code-reflection] RFR: [hat] DeviceType interface definition for private/local data structures [v3]

[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

Juan Fumero has updated the pull request incrementally with two additional commits since the last revision:

 - [hat] minor change
 - [HAT] mxm example using F16 fixed

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

Changes:
  - all: https://git.openjdk.org/babylon/pull/678/files
  - new: https://git.openjdk.org/babylon/pull/678/files/bdb3444a..4a650501

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=678&range=02
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=678&range=01-02

  Stats: 27 lines in 2 files changed: 11 ins; 3 del; 13 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