[foreign-memaccess+abi] RFR: 8292918: Split FunctionDescriptor into an interface and implementation(s)

[Per Minborg]

This PR splits the current FunctionDescription class into an interface and two internal implementing classes.

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

Commit messages:
 - Separate FunctionDescription

Changes: https://git.openjdk.org/panama-foreign/pull/713/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=713&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292918
  Stats: 396 lines in 2 files changed: 246 ins; 125 del; 25 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/713.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/713/head:pull/713

PR: https://git.openjdk.org/panama-foreign/pull/713