fix: resource lifetimes

[guybedford]

This fixes a few minor edge cases around resource handling, resolves #243, #249.

• We no longer null out Symbol.dispose when a resource has been disposed - instead we replace it with an empty function.
• We now explicitly track drops for low-level function borrows, and throw a resource error if any of the borrows were not dropped before the end of the function call.
• In the process, referring back to handle variables from earlier code output, I've updated the bindgen to use var over const where appropriate. In fact, it is known that using var has significant performance benefits over const in JS engines, so this can even be classed as a performance improvement.