Now a tez cent is 10_000L. All constants are now in the
Constants_repr module and expressed as multiples of one_cent.
Add new function Qty_repr.mul_exn to multiply `tez` by `int`
eg. `10 tez = Tez_repr.(mul_exn one 10)`
`10 cents = Tez_repr.(mul_exn one_cents 10)`
Remove `Tez.{to,of}_cents` and replace them with `Tez.{to,of}_mutez`.
- introduced `test/utils/test_lib.inc.sh` to simplify usage of
sandboxed node/client in the testsuite
- it reuses code from `./script/{node,client}_lib.inc.sh`
- use `wait_for_the_node_to_be_ready` to properly wait for the node to
be launched rather to use a fexed delay
- `test_multinode.sh` now launch 8 nodes.
