[code-reflection] RFR: Split OnnxRuntime into high-level and low-level generated code [v2]

[Maurizio Cimadamore]

> This PR reorganizes the code in the OnnxRuntime class. It now contains an higher-level implementation of the onnx C API, which is separate from the low-level bindings. These bindings are defined in the new `foreign` package, and have been mechanically generated with a tweaked version of `jextract` which:
> * omits some of the code that is not required (e.g. struct getters/setters)
> * make the generated code more usable for onnx use case -- meaning each struct field should have an "invoker" accessor, which fetches the struct pointer and then invokes it
> 
> The generated code is still quite big (OrtApi struct contains 10K LoC). I can make the bindings smaller if we only want to focus on the handful of functions we use today -- but in its current form, the low-level bindings are "complete" and support the full C API.
> 
> The main changes in OnnxRuntime are as follows:
> * calls to function pointers in OrtApi/OrtApiBase now are handled as calls to static wrappers generated by jextract.
> * there's no need to try/catch and wrap exceptions because the generated code does that
> * some of the explicit reinterpret calls with an explicit size have been replaced by generated `reinterpret` methods (which use the correct struct layout size)
> * some of the constants (logging level, API version) are now taken from the generated code
> 
> When doing this exercise, I uncovered an issue with the MHs for a couple of functions (`ReleaseEnv` and `ReleaseSession`) being incorrect -- the underlying function pointers for these are void (so no result) and they don't accept a trailing `ret` segment. Also calling `checkStatus` on them makes no sense (again, because these functions don't return a status).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Use jextract type constants
  Fix lifetime of output address params

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Changes:
  - all: https://git.openjdk.org/babylon/pull/323/files
  - new: https://git.openjdk.org/babylon/pull/323/files/5d8f9b03..fe746c01

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=323&range=01
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=323&range=00-01

  Stats: 25 lines in 1 file changed: 1 ins; 4 del; 20 mod
  Patch: https://git.openjdk.org/babylon/pull/323.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/323/head:pull/323

PR: https://git.openjdk.org/babylon/pull/323