new pretty printer

All strings should be properly escaped but some non-escaped versions
were leaking into error messages.

Functions like trust_anchors.summary() return strings including tabs and
newlines, so we do not escape those for human-readable output.

In case nothing (not even nil) is returned then do not print newline
after the zero-length message. This avoids unambiguous empty line in
output (is it an empty string which got printed?).

This change allows sandbox to pretty-print return values from functions
which return multiple values, e.g. future net.bufsize() from MR !1026.

This slightly changes table_print() output format.
table_print() output is not intended for machine consumption,
use krprint.serialize_lua() or JSON for that purpose.

Output from table_print is now a valid Lua expression
if the input contains only serializable data types
(number, string, bool, nil, table), which is nice for copy&pasting.

Functions etc. are also pretty-printed but cannot be deserialized.
Numbers are pretty-printed as well so their precision is reduced (as
compared to krprint.serialize_lua).

Code was copied from Lua sandbox, the old copy is to be removed once new
pretty-printer is finished.

At the moment it does not handle functions and other non-serializable
types.

Side-effect is that keys are grouped by their type, so numbers come
first and strings later..