[foreign-abi] RFR: 8247488: Add support for string helpers in CSupport

[Maurizio Cimadamore]

This is a relatively straightforward patch, which takes some of the code which jextract generates to help with string
conversion to/from Java and puts it into the recently added CSupport class.

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Commit messages:
 - Merge against latest version of code
 - Add string helpers to CSupport

Changes: https://git.openjdk.java.net/panama-foreign/pull/200/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/200/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247488
  Stats: 286 lines in 4 files changed: 175 ins; 110 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/200.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/200/head:pull/200

PR: https://git.openjdk.java.net/panama-foreign/pull/200