One-line whitespace-sensitive programs

[LightAndLight]

Required for #209 to work well.

I should be able to write "block" expressions like comp and case on a single line.

Possible syntaxes:

• comp { bind name <- readln; println "hi $name"; }
• comp; bind name <- readln; println "hi $name"; end

Ruled out:

• comp; bind name <- readln; println "hi $name"

  I think this style required too much lookahead to parse, e.g. in

  case x of; A -> comp; println "a1"; println "a2"; y -> println "b"
  

  y looks like a function call in a computation expression. The parser will only know it's in a pattern context by reaching ->, failing, then backtracking.