LibJS: Use known binding indices when creating new for-loop environments

When the initialization statement of a for-loop uses 'let', we must
create a new environment for each iteration of the for loop. The
bindings of the initialization statement are copied over to the new
environment. Since the bindings are created in the same order each time,
we can use that order to directly initialize the bindings and avoid any
O(n) lookups in this hot loop.

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