[plt-dev] mzlib/trace better prefixes

Attached is a patch that changes the prefixes that mzlib/trace prints 
depending on whether a function call is occuring or if a result is being 
returned. "<" is a function call and ">" is a return value. I find this 
is easier to read than the current ambiguous "|". If no one disagrees 
with the change in style then I will clean up the patch and commit it.

<(a 0)
< (b 0)
< <(c 0)
< < (d 0)
 > > 0
 > >1
 > 2
 >3
3

vs

|(a 0)
| (b 0)
| |(c 0)
| | (d 0)
| | 0
| |1
| 2
|3
3


Results become very ambiguous when the return value is a list that looks 
like a function call.

#lang scheme

(require mzlib/trace)

(define (foo x) `(foo ,x))
(trace foo)
(foo 2)

$ mzscheme n.ss
|(foo 2)
|(foo 2)
(foo 2)
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