Let-syntax support

[ValorZard]

Hi!
I’d like to take a stab at implementing a PR for this, but first I’d like to understand what I’m getting myself into.
Seems like once let-syntax is added, steel will be fully r5rs compatible, which is awesome.

Link to let-syntax:
https://standards.scheme.org/corrected-r7rs/r7rs-Z-H-6.html#TAG:__tex2page_sec_4.3.1

https://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let-syntax%29%29

[robinplace]

Hey @mattwparas I may take a stab at this to help familiarize myself with steel in general. I'm aware you're currently making good progress on adding a JIT to the compiler; are we at risk of ending up with conflicts if I dig in here?

[robinplace]

Oh I'm seeing lots of JIT PRs already merged. Looks like I can proceed with no worry of merge conflicts. That's great.

[mattwparas]

I don't think you'll run into many merge conflicts with the JIT work I'm doing with this if you give it a shot. Should be in pretty separate parts of the code base