[code-reflection] RFR: Replace use of MethodTypeDesc with FunctionType [v3]

[Paul Sandoz]

> Replace use of `MethodTypeDesc` with `FunctionType` in the core API.
> 
> This pushes `MethodTypeDesc` more towards the reflective operation usages. There are convenience methods added to `MethodTypeDesc` for conversion/resolution between `FunctionType`. However, it may that the right approach here is convenience methods for conversion/resolution between `java.lang.invoke.MethodType` (or sequences of `Class<?>`) and `FunctionType`, which means we can likely remove `MethodTypeDesc`.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Update Triton PoC

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

Changes:
  - all: https://git.openjdk.org/babylon/pull/22/files
  - new: https://git.openjdk.org/babylon/pull/22/files/fa2018bb..e1e64591

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=22&range=02
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=22&range=01-02

  Stats: 23 lines in 3 files changed: 2 ins; 4 del; 17 mod
  Patch: https://git.openjdk.org/babylon/pull/22.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/22/head:pull/22

PR: https://git.openjdk.org/babylon/pull/22