[code-reflection] RFR: Remove MethodTypeDesc

[Paul Sandoz]

Remove `MethodTypeDesc` and replace with usages of `FunctionType`, with functionality to convert between nominal description `java.lang.constant.MethodTypeDesc` (from which it can be resolved to `java.lang.invoke.MethodType`).

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Commit messages:
 - Remove MethodTypeDesc.

Changes: https://git.openjdk.org/babylon/pull/31/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=31&range=00
  Stats: 475 lines in 33 files changed: 51 ins; 279 del; 145 mod
  Patch: https://git.openjdk.org/babylon/pull/31.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/31/head:pull/31

PR: https://git.openjdk.org/babylon/pull/31