fmt/format-print and "containers" with only whitespace between delims

[sogaiu]

fmt/format-print appears to have difficulty with code strings such as:

• ( )
• @( )
• [ ]
• @[ ]
• { }
• @{ }

That is, there are no "elements" and there is at least one whitespace character.

An example invocation is:

repl:1:> (import spork/fmt)
# output elided
repl:2:> (fmt/format-print "[ ]")
error: match error at line 1, column 2
  in peg/match
  in make-tree [/home/user/bin/janet/lib/janet/spork/fmt.janet] on line 53, column 9
  in format-print [/home/user/bin/janet/lib/janet/spork/fmt.janet] on line 181, column 3
  in _thunk [repl] (tailcall) on line 2, column 1

This is with 714c216