Compiler support for specialised implicit value cells

Compiler support for specialised implicit value cells

 Enables the use of non-T stack TNs for closed-over dynamic-extent variables.

 SB!VM:PRIMITIVE-TYPE-INDIRECT-CELL-TYPE takes a primitive-type and returns
 NIL, or a list of 4 values:
  - the primitive type of the implicit indirect value cell
  - the SC of that cell
  - a function that takes the NODE, BLOCK, FP, VALUE and RESULT, and
    emits a reference to that cell.
  - a function that takes the NODE, BLOCK, FP, NEW-VALUE and VALUE, and
    emits a write to that cell.

 Some correctness tests to make sure codegen is correct (without verifying
 DXness).