scripts: Added CsvFfrac type

A simple float variant of the CsvFrac type:

- frac(1.5,2)  => 1/2 (50.0%)
- ffrac(1.5,2) => 1.5/2.0 (75.0%)

Useful for `make bench-widths` (previously make bench-bus), where we
want to find the average buffer utilization:

  probe            readed              progged                 erased
  b_rbyd+create   1.0/1.0 (100.0%)  13.8/256.0 (5.4%)        ∞/4096.0 (∞%)
  b_rbyd+delete     ∞/1.0 (∞%)         ∞/256.0 (∞%)          ∞/4096.0 (∞%)
  b_rbyd+fetch    1.0/1.0 (100.0%)     ∞/256.0 (∞%)          ∞/4096.0 (∞%)
  b_rbyd+lookup   1.0/1.0 (100.0%)     ∞/256.0 (∞%)          ∞/4096.0 (∞%)
  b_rbyd+usage      ∞/1.0 (∞%)         ∞/256.0 (∞%)          ∞/4096.0 (∞%)
  b_wt_seq+w      1.0/1.0 (100.0%)  31.7/256.0 (12.4%)  4096.0/4096.0 (100.0%)
  b_wt_random+w   1.0/1.0 (100.0%)  15.3/256.0 (6.0%)   4096.0/4096.0 (100.0%)
  b_wt_logging+w  1.0/1.0 (100.0%)  15.4/256.0 (6.0%)   4096.0/4096.0 (100.0%)
  b_wt_many+w     1.0/1.0 (100.0%)  16.1/256.0 (6.3%)   4096.0/4096.0 (100.0%)
  TOTAL             ∞/1.0 (∞%)         ∞/256.0 (∞%)          ∞/4096.0 (∞%)

Now that we have 4 types, the cast matrix gets a bit complicated, but
this is side-stepped a bit by a custom __frac__ hook.

---

Some other tweaks to csv.py:

- Added CsvFold.type to typecheck folds _after_ we know the expr's final
  type.

- Adopted CsvFfrac as an output for most of the math functions/folds

- Stopped early termination of typechecking if we change type!

  This was broken: int(float(1.5) + int(1))

Makefile

@@ -661,15 +661,30 @@ bench-ops: $(BENCH_CSV)
 		$(SUMMARYFLAGS))
 
 ## Show average readed/progged/erased per width
-.PHONY: bench-bus
-bench-bus: SUMMARYFLAGS+=-Si
-bench-bus: $(BENCH_CSV)
+.PHONY: bench-widths
+bench-widths: SUMMARYFLAGS+=-Si
+bench-widths: $(BENCH_CSV)
 	$(strip ./scripts/csv.py $^ \
 		-bprobe='%(case)s+%(probe)s' \
 		-Fi='min(enumerate())' \
-		-freaded='avg(float(bench_readed)/float(bench_reads))' \
-		-fprogged='avg(float(bench_progged)/float(bench_progs))' \
-		-ferased='avg(float(bench_erased)/float(bench_erases))' \
+		-freaded="avg(ffrac( \
+			float(bench_readed)/float(bench_reads), \
+			max(1, $$( \
+				./scripts/bench.py -R$(BENCH_RUNNER) $(BENCHFLAGS) \
+						--list-implicit-defines \
+					| sed -n 's/^READ_WIDTH=\(.*\)/\1/p'))))" \
+		-fprogged="avg(ffrac( \
+			float(bench_progged)/float(bench_progs), \
+			max(1, $$( \
+				./scripts/bench.py -R$(BENCH_RUNNER) $(BENCHFLAGS) \
+						--list-implicit-defines \
+					| sed -n 's/^PROG_WIDTH=\(.*\)/\1/p'))))" \
+		-ferased="avg(ffrac( \
+			float(bench_erased)/float(bench_erases), \
+			max(1, $$( \
+				./scripts/bench.py -R$(BENCH_RUNNER) $(BENCHFLAGS) \
+						--list-implicit-defines \
+					| sed -n 's/^ERASE_WIDTH=\(.*\)/\1/p'))))" \
 		$(SUMMARYFLAGS))