RFR: 7903272: Make method handle methods private

[Per Minborg]

On Tue, 30 Aug 2022 14:42:45 GMT, Per Minborg <duke at openjdk.org> wrote:

> xxxx$MH methods should be declared private to reduce cluttering in end users' IDEs and elsewhere.
>  
>  This PR also solves esthetic issues in the generated code.

There is a middle way where we could put the native method handles in an inner class (e.g. MethodHandles) exposing these methods for the casual user while still keeping the most used API clean.

Before: `foo_h.bar$MH()`
After: `foo_h.MethodHandles.bar()`

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PR: https://git.openjdk.org/jextract/pull/67