Sequentialize parallel let-bindings

[j-hui]

The user may write:

let x = xdef
    y = ydef
xyscope

for brevity. Of course, this isn't a lazy language, so xdef cannot actually reference y and ydef cannot actually reference x, but so long as that is the case, this is semantically equivalent to writing:

let x = xdef
let y = ydef
xyscope

Indeed, LowerAst will only work on the second form but not the first. We need to implement a desugaring pass that does this for us.

Furthermore, something like:

let f x = if x { h x } else { () }
    g x = if x { h x } else { () }
    h x = if x { g x } else { () }
fghscope

can also be partially sequentialized:

let g x = if x { h x } else { () }
    h x = if x { g x } else { () }
let f x = if x { h x } else { () }
fghscope

This may make type-checking/inference much easier, depending on how mutual recursion is deal with, so this is something we would want done too.

In summary:

• sequentialize parallel variable bindings
• best-effort sequentialize parallel function bindings

[j-hui]

This is being dealt with in #103