more than 10 operands in `asm'

[Jamie Lokier]

Two more syntax possibilities:

  - Retain the old meaning of %10 when there are fewer than ten
    operands, but treat it as operand #10 when there are ten or more
    operands.  This is nice because it's definitely backwards compatible.

    To avoid too many surprises, I'd make the rule: if there are fewer than
    10 operands, %[0-9] is an operand and %[0-9][0-9]+ elicits a warning.
    If there are 10 or more operands, %[0-9]+ is an operand and all digits
    are consumed.  A number out of range is an error.

  - Extend the syntax to %{[0-9]+} for multi-digit operands.

-- Jamie