corrections to reference implementation

[William D Clinger <will@xxxxxxxxxxx>]

The current reference implementation of SRFI 43 blows up on
several trivial cases that, according to the specification
itself, would appear to be legal.  Examples:

    (vector-copy '#() 0)
    (vector-copy! (vector) 0 '#())
    (vector-reverse-copy! (vector) 0 '#())

All three problems arise from premature index checking.  I have
rewritten those procedures to match the SRFI's specification:

https://trac.ccs.neu.edu/trac/larceny/browser/trunk/larceny_src/lib/SRFI/srfi-43.sch

Will