meas: correctly render SI prefixes for negative numbers

the previous implementation treated negative numbers as effectively
having unknown magnitude, rendering them without any adjustment.

crates/coremem/src/meas.rs

@@ -118,7 +118,7 @@ enum SiScale {
 impl SiScale {
     fn for_value(v: f32) -> Self {
         use SiScale::*;
-        match v {
+        match v.abs() {
             v if v < 1e-12 => Unit,
             v if v < 1e-9  => Pico,
             v if v < 1e-6  => Nano,