Add Parsetree-level Option stdlib optimizations (forEach/map/flatMap)

[cknitt]

This PR adds a compiler-side optimization for the most important functions from the standard library's Option module: Option.forEach/map/flatMap.

• At the Parsetree stage, we rewrite these into the same switch structure a handwritten version would yield, whenever the callback is either a literal lambda with a simple variable binder or an inlineable identifier. More complex callbacks stay as calls so evaluation order and side-effects remain untouched.
• Because the rewrite runs ahead of type checking, the existing pipeline still inserts Primitive_option.valFromOption exactly as before, so the generated JavaScript stays identical. The trade-off is that we can only recognize calls spelled explicitly as Option.* or Stdlib_Option.*; aliases or locally opened variants aren’t rewritten.
• Typedtree and Lambda remain untouched. Threading a rewrite through Typedtree would be far more invasive than we need, and once we reach Lambda the primitive-versus-boxed distinction this optimization relies on has already disappeared.
• The change is covered by the comprehensive tests in tests/tests/src/option_stdlib_optimization_test.res. The first test in that file gives a good example of how much the compiler output for long pipe chains is simplified.

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commit: 50dbc3c

[cknitt]

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