[foreign-memaccess+abi] RFR: Deduplicate layouts and simplify value layout implementations [v2]

[Per Minborg]

> This PR proposes adding deduplication of the immutable value layout where one provides a value that is equal to the one already at hand. For example, `JAVA_INT.withBitAlignment(32)` will return the same object. There is no general caching mechanism that guarantees a layout will never repeat. 
> 
> Another part of this PR relates to simplifying value layouts so they only have a single constructor. This will reduce size slightly. Related to this is the replacement of certain magic numbers which are now represented by constants.

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

 - Rollback changes in constructor
 - Rollback changes not related to dedup

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/795/files
  - new: https://git.openjdk.org/panama-foreign/pull/795/files/90e49596..939b0c94

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=795&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=795&range=00-01

  Stats: 69 lines in 1 file changed: 50 ins; 0 del; 19 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/795.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/795/head:pull/795

PR: https://git.openjdk.org/panama-foreign/pull/795