Start on ByteCode interpreter

[jasonwilliams]

This does run!

Nodes implement CodeGen which generates instructions onto a stack held in Context.
The VM will interpret the instructions from Context.

There are some issues:

• only basic instructions are added, but im working off https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/Bytecode for now it should be easy to add more in.
• The Stack is a Vec, this isn't ideal (we may be able to live with it for now) but the stack should really be a fixed sized array. This isn't possible because Value can't be copied in there as it holds Rc and Gc values. Can we have fixed-sized Values that hold a pointer?
• I needed to copy the Instruction vec, i couldn't find a way to pass in both ctx and ctx.instructions together without it moaning about "mutable borrowing" twice.
• I need to change boa_cli back to the normal run.

As most of this is behind a feature flag it would be good to merge parts of it so we can work on as the main interpreter is active. Then one day we can switch over.

for

2 + 2

The instructions generated are

[boa/src/context.rs:689] &self.instruction_stack = [
    Int32       2,
    Int32       2,
    Add,
]
4

We can optimize even further here and just do the addition at CodeGen level and throw 4 onto the stack

Fixes #167
@Razican @HalidOdat

[codecov]

Codecov Report

Merging #854 into master will decrease coverage by 0.24%.
The diff coverage is 32.63%.

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
- Coverage   59.24%   58.99%   -0.25%     
==========================================
  Files         157      160       +3     
  Lines       10037    10132      +95     
==========================================
+ Hits         5946     5977      +31     
- Misses       4091     4155      +64     

Impacted Files	Coverage Δ
boa/src/lib.rs	86.36% <ø> (ø)
boa/src/syntax/ast/node/operator/bin_op/mod.rs	66.08% <0.00%> (-4.29%)	⬇️
boa/src/syntax/ast/node/statement_list/mod.rs	42.22% <0.00%> (-6.50%)	⬇️
boa/src/vm/compilation.rs	0.00% <0.00%> (ø)
boa/src/vm/instructions.rs	0.00% <0.00%> (ø)
boa/src/vm/mod.rs	0.00% <0.00%> (ø)
boa_cli/src/main.rs	33.33% <ø> (ø)
boa/src/context.rs	57.38% <6.25%> (-3.71%)	⬇️
boa/src/builtins/object/mod.rs	64.86% <73.17%> (+3.18%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 36b7c31...72b8680. Read the comment docs.

[jasonwilliams]

continuing here #860