compiler.let_bindings_parser()
      Return a parser for let bindings.
The following program adds a new control strcuture to Slogan, similar to let expression.
      The form is: doit <expression> where (<bindings>). The expression will be evaluated in a context
      where the variables introduced by bindings are visible. For parsing bindings the let_bindings_parser
      function is used, instead of writing a custom parser.
declare macro
 doit $body where $bindings
 => compiler.let_($bindings, $body),
 [$bindings:compiler.let_bindings_parser]
doit a + b where (a = 100, b = 200)
// 300