quasilit: expose access to field formatting.

This is the second round of changes in perparation for
compiling the string quasiliterals special form.

Having split format_fields into a lower level do_format_field
and format_field, we now provide two functions which allow
Lisp code to call do_format_field.

The sys:fmt-simple function has fixed arguments for
the field width/alignment, indexing/slicing, separator
and plist.  Any of them can be defaulted with nil. This
function will be useful when the quasiliteral specifies
modifiers that are simple literals or constant expressions.
The sys:fmt-flex function takes a variable number of
arguments, after the object and the plist. This function
has to scan through the arguments and classify them
by type: a string is a separator, an integer is a width
and left/right alignment and so on. The compiler will use
sys:flex when format field modifiers are present whose
arguments contain expressions that get evaluated.

* eval.c (fmt_simple, fmt_flex): New static functions.
(eval_init): sys:fmt-simple and sys:fmt-flex registered.

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