[jow-]

Support array and object destructuring in let/const declarations and in assignment expressions, including nested patterns, default values, holes, computed keys, member targets and rest elements:

let [a, {b, c: [d = 1]}, ...rest] = src;
const {x, y: z, w = 5, ...others} = obj;
[arr[i], o.k, ...o.tail] = src;
({a, [key]: b = 3, ...others} = obj);

Assignments evaluate to the assigned value and may appear wherever an assignment expression can. The initializer is evaluated first, then the targets and the loaded values of each item in turn; declared variables are inaccessible while the initializer and defaults are compiled, yielding the usual temporal dead zone errors.

An array/object literal that starts an assignment expression is a pattern if the matching closing bracket is followed by =. This is determined by the new uc_lexer_skip_balanced(), which skips raw text up to the closing bracket (recognizing only strings, template literals, regexps and comments), without compiling anything or affecting lexer/parser state — so the nature of the expression is known ahead of compiling it, without backtracking. Sources gain uc_source_read_at() (read-ahead peek at arbitrary offsets) and uc_source_resync() (resume sequential reading).

To generate code in source order despite the initializer-evaluates-first semantics, pattern code is emitted ahead of initializer code with jumps to run them in the right order; nested pattern defaults use the same scheme.

Rest elements reuse existing machinery instead of adding instructions: the container is copied with I_NARR/I_NOBJ, merged with I_MARR/I_MOBJ exactly like a literal spread (so an unsuitable container is reported the same way), and the already-bound items are taken off the copy with internal splice()/rawdelete() calls emitted as I_STCALL. Both work on plain copies, so no metamethods are dispatched. No new VM instruction is introduced.

Stacked on #509 (vm: invoke standard library functions internally, add typeof and throw), which provides I_STCALL; merge that first.