fix: bind a const pattern element to its default rather than assigning it

[marevol]

Stacked on #58.

A const binding whose default was actually taken failed at run time:

const [a = 7] = [];       // TypeError: Assignment to constant "a"
const { x = 8 } = {};     // same
const [a = 7] = [1];      // 1, fine - the default was not needed

Cause

declareBindings emitted the declaration and then filled the default in with a follow-up assignment, if (a === undefined) a = 7;. A const binding cannot be assigned to a second time, so the guard threw whenever it fired. let and var were unaffected, which is why the default path looked like it worked.

Fix

The default now goes into the initializer instead, as a conditional. The value is read once into a temporary first and the temporary is what gets tested and used, rather than testing the read expression and then reading it again — a getter is still invoked exactly once, as it was with the guard.

Found while adding patterns to for-of (#58); it is not specific to loops and reproduces on a plain declaration.

Verification

./gradlew build testOptimistic testPessimistic

suite	before	after
test	666, 0 fail	666, 0 fail
testOptimistic	1718, 0 fail	1718, 0 fail
testPessimistic	1718, 0 fail	1718, 0 fail