Support the use of do as a connective in UCS

[chengluyu]

Main features:

• Keyword do can appear in place of then, and the default value of its UCS expression will be replaced with UnitLit.
• Keyword do can appear in a split of UCS to introduce expressions whose results are not needed between branches.

Relevant changes:

• Keyword do has the same left and right precedence as then and can be parsed as an infix operator.
• Add two Sets to Desugarer tracking the usage of do and then. If they are all used in the same UCS expression, an error will be reported.
• Add test file ucs/syntax/Do.mls, which shows the semantic difference between do and then.

Not so relevant changes:

• Add a unified apply to Desugarer. It should be called no matter we want to desugar case, if, or UCS shorthands.
• Add kwLoc: Opt[Loc] field to IfLike and Case for better error reporting.

[LPTK]

Nice change! There are a few places to update that currently use while imperatively but with then because we didn't have do at the time.

[chengluyu]

Can you please squash this PR when you merge?

[LPTK]

All right, let's merge this.

Though I am not quite sure about the do situation. I don't like that it's currently overloaded, making things a bit difficult to parse sometimes, as in:

while
  do log(i)
  i < 3 do set i += 1

or

while
  do print(ls)
  ls is Cons(h, tl) do set ls = tl

Here the two uses of do serve quite different purposes, making things read a bit awkward.