Change schema for {100..50..-5}.

It avoids both negative numbers and optional integers.

Since it's a lossless syntax tree, we want to distinguish {1..5} from
{1..5..0}.  The latter is invalid!

osh/osh.asdl

@@ -47,6 +47,9 @@ module osh
  -- NOTE: identical strings can shared, if we care.
  token = (id id, string val, int? span_id)

  -- Optional step for {100..50..-15}
  braced_step = (int val, int negated)

  bracket_op = 
    WholeArray(id op_id)  -- * or @
  | ArrayIndex(arith_expr expr)
@@ -81,9 +84,9 @@ module osh
    -- {a,b,c}
  | BracedAltPart(word* words)
    -- {1..10} or {1..10..2}
  | BracedIntRangePart(int start, int end, int? step)
  | BracedIntRangePart(int start, int end, braced_step? step)
    -- {a..f} or {a..f..2} or {a..f..-2}
  | BracedCharRangePart(string start, string end, int? step)
  | BracedCharRangePart(string start, string end, braced_step? step)
	-- extended globs are parsed statically, unlike globs
  | ExtGlobPart(token op, word* arms)