RFR: 7903190: jextract runtime helper issues [v2]

[Athijegannathan Sundararajan]

> * using varargs flag during jextract time rather than at runtime
> * avoiding redundant null checks in generated code
> * avoiding String to MethodType conversion in RuntimeHelper.upcallStub method

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last revision:

  adding --version option to README.md

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/33/files
  - new: https://git.openjdk.java.net/jextract/pull/33/files/55d61cf7..53c86ff5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=33&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=33&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jextract/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/33/head:pull/33

PR: https://git.openjdk.java.net/jextract/pull/33