Make Rc lists clonable without cloning any of its elements.

[01mf02]

This PR makes reference-counted lists clonable without cloning any of its elements.

Reference-counted lists are used to store the contents of bound variables in jaq.
As they are part of the filter execution context, they are very frequently cloned.
So far, cloning such a list would always clone the head of list, in addition to cloning an Rc-pointer to the tail of the list.
After this change, cloning only clones an Rc-pointer to the list.

The performance implication of this change is quite favourable:

Benchmark	n	Before	After
empty	512	630	650
bf-fib	13	400	380
reverse	1048576	30	30
sort	1048576	100	100
group-by	1048576	440	390
min-max	1048576	180	180
add	1048576	460	420
kv	131072	140	150
kv-update	131072	150	150
kv-entries	131072	590	560
ex-implode	1048576	750	740
reduce	1048576	720	690
try-catch	1048576	150	130
tree-flatten	17	450	420
tree-update	17	280	260
tree-paths	17	1250	1210
to-fromjson	65536	30	30
ack	7	560	520