Improved let syntax

[rolyp]

Following on from #229, we should probably make some effort to make the let and recursive let syntax more familar. We should be able to support:

• nested let for the non-recursive form as well as the recursive form, so we can write let x = 5; y = 6; ... – although one challenge here is that it will not be obvious why you can’t add a recursive definition to such a block, but instead have to start a new (recursive) let block (we could reintroduce the rec keyword to make this explicit)
  • in “modules”
  • in let expressions
• in as the delimiter in expressions
• but not in “pretend modules”
  • replace many by sepBy and some by SepBy1, turning semicolon into delimiter rather than terminator – needs sepBy1_try
  • in to separate list of defs from expr in defsExpr
  • semicolon to terminate list of defs in module – needs sepBy_try
• replace letDefs and recDefs parser by new defsExpr parser, which folds a list of variable and function definitions and a terminal expression into a tree of Let or LetRec expressions
• keyword “blah” should be a fatal error if “blah” is not a keyword

None of this is as straightforward as one would like, though, especially if both familiarity and non-verbosity are desiderata.