[foreign-memaccess+abi] RFR: 8292038: Split factories to create sequence layouts [v2]

[Per Minborg]

> This PR splits the `MemoryLayout::sequence` method into two separate overloads:
> 
> 1. Explicit `elementCount`
> 2. Inferred `elementCount`

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

  Improve JavaDoc

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/704/files
  - new: https://git.openjdk.org/panama-foreign/pull/704/files/20b76c9f..dd1a91d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=704&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=704&range=00-01

  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/704.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/704/head:pull/704

PR: https://git.openjdk.org/panama-foreign/pull/704