This PR implements an optimizer, It currently implements the [constant folding optimization][cfo]. this optimization is responsible for "folding"/evaluating constant expressions.
For example:
```js
let x = ((1 + 2 + -4) * 8) << 4
```
Generates the following instruction(s) (`cargo run -- -t`):
```
000000 0000 PushOne
000001 0001 PushInt8 2
000003 0002 Add
000004 0003 PushInt8 4
000006 0004 Neg
000007 0005 Add
000008 0006 PushInt8 8
000010 0007 Mul
000011 0008 PushInt8 4
000013 0009 ShiftLeft
000014 0010 DefInitLet 0000: 'x'
```
With constant folding it generates the following instruction(s) (`cargo run -- -t -O`):
```
000000 0000 PushInt8 -128
000002 0001 DefInitLet 0000: 'x'
```
It changes the following:
- Implement ~~WIP~~ constant folding optimization, ~~only works with integers for now~~
- Add `--optimize, -O` flag to boa_cli
- Add `--optimizer-statistics` flag to boa_cli for optimizer statistics
- Add `--optimize, -O` flag to boa_tester
After I finish with this, will try to implement other optimizations :)
[cfo]: https://en.wikipedia.org/wiki/Constant_folding
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