flatMap reads the array its callback gives and leaves it as it was - #99

What

flatMap no longer empties the array its callback returns. It copies the elements out, as flat does, where it moved them.

Why

A callback that returned an array it kept had it drained on the first call: the same array for every element, or a row of a table.

const pair: number[] = [7, 8];
[1, 2, 3].flatMap((v: number, i: number) => pair);  // was "7,8"; now "7,8,7,8,7,8"
// pair was left empty; now it is still [7, 8]

Change

• src/generics/lib.generics.ts: the loop in __Array.flatMap pushes each element of the returned array. Array<T>.flatMap calls it, so it is fixed too.
• tests/array_flatMap_shared.ts (new): a shared array, rows of a table, and the Array<T> class. It failed before the fix.

For the reviewer

• -mm=own: the new test is marked // not own and skipped there. The compiler refuses a callback that returns an array it does not own ("borrows a captured variable"), so the bug could not occur under that model. flatMap was already @sharesElements, so what own accepts is unchanged.
• The elements were moved since process and fs; Array flat/flatMap/toSpliced, Map.groupBy, String.fromCodePoint #90, so this is not a regression from Quadratic paths made linear; Headers.get, regex crashes and five wrong results fixed #96, which only made the move linear.

Testing

• Release, Windows x64: the whole suite passes for gc, rc and none, compiled and under the JIT, and for own under the JIT; the two flat tests were also run compiled for own.
• Not run locally: debug builds, Linux, x86 and wasm.

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