style fallbacks

[Bill McClain <wmcclain@salamander.com>]

Are style fallbacks a supported part of the system, or are they local to
type-buy.tex?

This typescript doesn't work because of missing Slant styles:

\starttypescript [serif] [charter] [name]
  \definefontsynonym [Serif]           [Charter-Roman]
  \definefontsynonym [SerifBold]       [Charter-Bold]
  \definefontsynonym [SerifItalic]     [Charter-Italic]
  \definefontsynonym [SerifBoldItalic] [Charter-Bold-Italic]
\stoptypescript

Adding \usetypescriptfile[type-buy] at the top and
\usetypescript[serif][fallback] within the typescript fixes that
problem, but the job still fails because of missing SerifCaps
definition, which isn't in the fallback typescript.

Adding this fixes it:

\starttypescript [serif] [fallback]
  \definefontsynonym [SerifCaps] [Serif]
\stoptypescript

Or, I could add dummy SerifCaps to the definition. Using the fallback
mechanism is more elegant. Is there a reason SmallCaps should be treated
differently than the Slant styles?

-Bill