[PFA 4/n] Optimize constant pre-bound arguments

[arnaud-lb]

The branch includes GH-22785, but only the commit "PFA: Optimize constant pre-bound arguments" is relevant for this PR. (Edit: branch was rebased and doesn't include GH-22785 anymore.)

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Currently PFA pre-bound arguments are bound to the generated closure's lexical vars:

function f($a, $b) {}
$f = f(1, ?);

// Generates:
$tmp = 1;
$f = function ($b) use ($tmp) {
    return f($tmp, $b);
};

In this PR we detect which pre-bound arguments are constant and burn them into the generated closure instead:

function f($a, $b) {}
$f = f(1, ?);

// Generates:
$f = function ($b) {
    return f(1, $b);
};

This reduces the overhead during both instantiation and invocation of PFAs.

[iliaal]

Looks good!