1.0.19.7: refactor stack allocation decisions

1.0.19.7: refactor stack allocation decisions

 * Remove SB-C::STACK-ALLOCATE-* policies.

 * Obey DYNAMIC-EXTENT declarations if SB-EXT:*STACK-ALLOCATE-DYNAMIC-EXTENT*
   is true (the default), with the following exceptions:

    ** Value cells are not stack allocated.

    ** Vectors that may be longer then a single page are stack
       allocated only in SAFETY 0 policies.

 * New declaration: SB-INT:TRULY-DYNAMIC-EXTENT. Always stack-allocates,
   regardless of SB-EXT:*STACK-ALLOCATE-DYNAMIC-EXTENT*. Also causes stack
   allocation of value cells and potentially large vectors.

   Used exclusively inside SBCL.

 * Move STACK-ALLOCATE-RESULT optimizers from backends to
   src/compiler/generic/vm-ir2tran.lisp.

 * Documentation.