Update special constants specification.

[MatthewFluet]

Minor update to the specification of special constants:

• a binary integer requires an 0b prefix
• sequences of digits and underscores should neither start nor end with an underscore.

These correspond to the support in SML/NJ and MLton. In particular, they rule out

0wx_FF_FF

where an underscore immediately follows the prefix (much as an underscore cannot immediately precede or follow . or e in a real constant).