[foreign-memaccess+abi] RFR: ValueLayouts: Improve test coverage and more [v2]

[Per Minborg]

> This PR:
> 
> * Adds tests to `ValueLayout` methods
> * Simplifies an `equals()` method
> * Changes from exception throwing to assert for parameters that cannot be changed client-side

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Update after comments

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/786/files
  - new: https://git.openjdk.org/panama-foreign/pull/786/files/5cdf3526..5336e0f8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=786&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=786&range=00-01

  Stats: 42 lines in 2 files changed: 19 ins; 5 del; 18 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/786.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/786/head:pull/786

PR: https://git.openjdk.org/panama-foreign/pull/786