[Python-3000] Substantial rewrite of PEP 3101

[Eric V. Smith]

 > Formatter Creation and Initialization
 >
 >     The Formatter class takes a single initialization argument, 'flags':
 >
 >         Formatter(flags=0)
 >
 >     The 'flags' argument is used to control certain subtle behavioral
 >     differences in formatting that would be cumbersome to change via
 >     subclassing. The flags values are defined as static variables
 >     in the "Formatter" class:
 >
 >         Formatter.ALLOW_LEADING_UNDERSCORES
 >
 >             By default, leading underscores are not allowed in identifier
 >             lookups (getattr or getitem).  Setting this flag will allow
 >             this.
 >
 >         Formatter.CHECK_UNUSED_POSITIONAL
 >
 >             If this flag is set, the any positional arguments which are
 >             supplied to the 'format' method but which are not used by
 >             the format string will cause an error.
 >
 >         Formatter.CHECK_UNUSED_NAME
 >
 >             If this flag is set, the any named arguments which are
 >             supplied to the 'format' method but which are not used by
 >             the format string will cause an error.

I'm not sure I'm wild about these flags which would have to be or'd
together, as opposed to discrete parameters.  I realize have a single
flag field is likely more extensible, but my impression of the
standard library is a move away from bitfield flags.  Perhaps that's
only in my own mind, though!

Also, why put this in the base class at all?  These could all be
implemented in a derived class (or classes), which would leave the
base class state-free and therefore without a constructor.

 > Formatter Methods
 >
 >     The methods of class Formatter are as follows:
 >
 >         -- format(format_string, *args, **kwargs)
 >         -- vformat(format_string, args, kwargs)
 >         -- get_positional(args, index)
 >         -- get_named(kwds, name)
 >         -- format_field(value, conversion)

I've started a sample implementation to test this API.  For starters,
I'm writing it in pure Python, but my intention is to use the code in
the pep3101 sandbox once I have some tests written and we're happy
with the API.