compiler: register-compacting optimization.

This work addresses the following issues, which cause compiled
functions to use more stack space than necessary.

Firstly, the compiler doesn't allocate registers tightly.
Every closure's registers can start at t2, but this isn't
done. Secondly, data flow optimizations eliminate registers,
leaving gaps in the register allocation. Code ends up strange:
you may see register t63 used in a function where the next
highest register below that is t39: evidence that a large
number of temporary variables got mapped to registers and
eliminated.

In this change, an optimization is introduced, active at
*opt-level* 6 which compacts the t registers for every
closure: every closure's registers are renumbered starting
from t2. Then, each closure' generating close instruction
is also updated to indicate the accurate number of registers
ensuring no space is wasted on the stack when the closure
is prepared for execution.

* stdlib/compiler.tl (compiler optimize): At optimization
level 6, insert a call to basic-blocks compact-tregs
just before the instruction are pulled out and put through
the late peephole pass.

* stdlib/optimize.tl (basic-block): New slot, closer.
This is pronounced "clozer", as in one who closes.
For any basic block that is the head of a closure (the entry
point ito the closure code), this slot is set to point
to the previous block: the one which ends in the close
instruction which creates this closure: the closer. This is
important because the close instruction can use t registers
for arguments, and those registers belong to the closure.
Those argument registers must be included in the renaming.
(basic-blocks): New slots closures and cl-hash. The former
lists the closure head basic blocks; all the basic blocks
which are the entry blocks of a closure. The cl-hash
associates each head block with a list of all the blocks
(including the head block).
(basic-blocks identify-closures): New method. This scans the
list of blocks, identifying the closure heads, and associating
them with their closers, to establish the bb.closure list.
Then for each closure head in the list, the graph is searched
to find all the blocks of a closure, and these lists are put
into the bb.cl-hash.
(basic-block fill-treg-compacting-map): This method scans
a basic block, ferreting out all of it t registers, and
adds renaming entries for them into a hash that is passed in.
If the block is the head of a closure, then the close
instruction of the block's closer is also scanned for t
registers: but only the arguments, not the destination
register.
(basic-block apply-treg-compacting-map): This method
renames the t register of a block using the renaming map.
It follows the registers in the same way as
fill-treg-compacting-map, and consquently also goes into
the close instruction to rename the argument registers, as
necessary. When tweaking the close instruction, it also
updates the instruction's number-of-tregs field with the
newly calculated number, which comes from the map size.
(basic-blocks compact-tregs): This method ties it together,
using the above methods to identify the basic blocks belonging
to closures, build register renaming maps for them, and hen
apply the renaming.

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