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
For each js script in the bench_scripts folder, we create three benchmarks:
Parser => lexing and parsing of the source code
Compiler => compilation of the parsed statement list into bytecode
Execution => execution of the bytecode in the vm
The idea is to check the performance of Boa in different scenarios.
Different parts of Boa are benchmarked separately to make the impact of local changes visible.
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