scripts: csv.py: Added saturate function

This is useful for enforcing an upper-bound on fracs based on their
total component.

Technically possible via decomposing + min + recomposing, but... Well
which one do you think is easier?

- saturate(x)
- frac(min(max(int(x), 0), total(x)), total(x))

And this assumes x is easily available and not some other expr (though
chaining csv.py could work around that).

---

The motivation for this was `make bench-widths`, where one read
benchmark could ruin the entire column due to introducing infinities.
Now:

  make bench # (squished a bit)
  probe           readed               progged                  erased
  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%)
  b_rt_seq+r     1.0/1.0 (100.0%)  256.0/256.0 (100.0%)  4096.0/4096.0 (100.0%)
  b_rt_random+r  1.0/1.0 (100.0%)  256.0/256.0 (100.0%)  4096.0/4096.0 (100.0%)
  b_rt_many+r    1.0/1.0 (100.0%)  256.0/256.0 (100.0%)  4096.0/4096.0 (100.0%)
  TOTAL          1.0/1.0 (100.0%)  149.0/256.0 (58.2%)   4096.0/4096.0 (100.0%)
  #                                  ^- notably not infinity

scripts/csv.py

@@ -1099,6 +1099,18 @@ class CsvExpr:
             else:
                 return v.b
 
+    @func('saturate', 'a')
+    class Saturate(Expr):
+        """Limit to total part of a fraction"""
+        def eval(self, fields={}, state=None):
+            v = self.a.eval(fields, state)
+            if not hasattr(v, '__frac__'):
+                return v
+            elif isinstance(v, CsvFrac):
+                return CsvFrac(min(max(v.a, CsvInt(0)), v.b), v.b)
+            elif isinstance(v, CsvFfrac):
+                return CsvFfrac(min(max(v.a, CsvFloat(0)), v.b), v.b)
+
     @func('abs', 'a')
     class Abs(Expr):
         """Absolute value"""