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

[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`.

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Commit messages:
 - Use FunctionType.

Changes: https://git.openjdk.org/babylon/pull/22/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=22&range=00
  Stats: 406 lines in 29 files changed: 61 ins; 54 del; 291 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