[foreign-jextract] RFR: 8243978: jextract should generate separate classes with static util methods for each struct, union

[Athijegannathan Sundararajan]

JavaSourceBuilder refactored. HeaderBuilder, StructBuilder are added.

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

Commit messages:
 - 8243978: jextract should generate separate classes with static util methods for each struct, union

Changes: https://git.openjdk.java.net/panama-foreign/pull/133/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/133/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8243978
  Stats: 480 lines in 10 files changed: 276 ins; 144 del; 60 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/133.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/133/head:pull/133

PR: https://git.openjdk.java.net/panama-foreign/pull/133