RFR: 8297733: Refactor Cast binding to enum

[Jorn Vernee]

Refactor the cast binding to an enum, which clearly enumerates all supported conversions.

This also fixes a bug where `java/foreign/normalize/TestNormalize` was failing when running in interpreted mode (`-Djdk.internal.foreign.DowncallLinker.USE_SPEC=false`), for conversions from `int` to `byte` because the `explicitCast` method handle that was used only considers the least significant bit, while we want to look at all of the least significant byte.

As mentioned in the JBS issue, doing this will also make it easier going forward to support multiple conversions with the same from and to types, but requiring different semantics.

Testing: jdk_foreign test suite.

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Depends on: https://git.openjdk.org/jdk/pull/11019

Commit messages:
 - Refactor cast operator to enum

Changes: https://git.openjdk.org/jdk/pull/11397/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11397&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297733
  Stats: 82 lines in 2 files changed: 56 ins; 14 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/11397.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11397/head:pull/11397

PR: https://git.openjdk.org/jdk/pull/11397