[foreign-memaccess+abi] RFR: Improve exception messages and simplify LayoutPath [v7]

[Per Minborg]

> This PR proposes to improve the exception messages in the internal `LayoutPath` class and also do some code simplifications.

Per Minborg has updated the pull request incrementally with three additional commits since the last revision:

 - Use distinct methods for type check
 - Revert back to the old reref logic
 - Remove Stable annotations

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/886/files
  - new: https://git.openjdk.org/panama-foreign/pull/886/files/32fe0526..c992f3bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=06
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=886&range=05-06

  Stats: 29 lines in 1 file changed: 10 ins; 8 del; 11 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/886.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/886/head:pull/886

PR: https://git.openjdk.org/panama-foreign/pull/886