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

[Per Minborg]

This PR splits the `MemoryLayout::sequence` method into two separate overloads:

1. Explicit `elementCount`
2. Inferred `elementCount`

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Commit messages:
 - Split sequence factories into two

Changes: https://git.openjdk.org/panama-foreign/pull/704/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=704&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292038
  Stats: 46 lines in 4 files changed: 21 ins; 13 del; 12 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