runtime: contiguous stacks

runtime: contiguous stacks

Each Go routine gets a single, contiguous stack instead
of the current system of chained stacklets.
We grow stacks by allocating a new, larger one
and copying the stack over.  Internal pointers in the
stack are adjusted appropriately.  (We're guaranteed by the
compiler that nothing external points into the stack,
at least for Go code.)

This implementation is simpler than the chained stacks and
won't have pathological stack splits (where calls across
a stacklet boundary cause stacklets to be allocated and
freed very quickly).

A lot of the defer/panic/recover mechanism has been changed
because vararg calls need to be done in a different fashion.

This implementation will require accurate pointer/notpointer
maps of every frame on the stack.  We don't have this info yet...

Can't handle cgo yet.