Supervisor channels are a simple concept to more efficiently
enable dynamic, structure concurrency. When a top-level fiber
completes (or errors), it will push itself to it's supervisor
channel if it has one (instead of printing a stacktrace). This
let's another fiber poll a channel and "supervise" a set of fibers.
Keep a separate stack for tagged references. May cause pegs to
use more memory but makes the backref and backmatch features much more
powerful.
Also disables the second stack if backref and backmatch are not used in the peg.
Does not require all sorts of signal handling code
that is not thread-safe and can "steal processes".
However, there is a much simpler way to add this functionality
by creating a new stream and thread for each subprocess when it is
waited on. This is perhaps _slightly_ less efficient but oh so much
simpler, since we can reuse all of our concepts from streams and there
is no need to implement a whole system around the selfpipe.
This lets Janet be a better unix citizen and lets Ctrl-C
raise an interrupt. Trying to make Janet behave superficially
like a shell by overriding Ctrl-C is not helpful.
The current implementation will have quadratic behaviour for already
sorted arrays because it picks the last element as pivot. In an sorted
array this splits the array repeatedly into the biggest value and all
other values.
The implementation in this commit uses the *median of three* as pivot.
`janet -e "(sort (range 10000))"` to reproduce quadratic behaviour.
`seed ^= hash_value(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);`
from https://www.boost.org/doc/libs/1_35_0/doc/html/boost/hash_combine_id241013.html
The current way of combining hashes peforms poorly on hash values of
numbers. Changing the way hashes are combined canlead to a significant speed up:
```
time janet_new -e '(def tbl @{}) (loop [x :in (range 1000) y :in (range 1000)] (put tbl {0 x 1 y} true))'
3.77s user 0.08s system 99% cpu 3.843 total
time janet_orig -e '(def tbl @{}) (loop [x :in (range 1000) y :in (range 1000)] (put tbl {0 x 1 y} true))'
48.98s user 0.15s system 99% cpu 49.136 total
```
