core: commands that take an index now use integer expressions

This means that instead of just [list index 2+1], we can now do
[list index end-$n*2+1]

This applies to:

    lindex, linsert, lreplace, lset, lrange, lsort, regexp, regsub
    string index,first,last,range

Also add tests for both direct integer expressions and indexes.

Still needs doc update.

Signed-off-by: Steve Bennett <steveb@workware.net.au>