Fix parsing logic after first whitespace

The way the parse was implemented accepted additional numeric characters
or `.` after the first valid `f64` literal but ignored them.

This permitted more strings that are invalid following a strict grammar
for byte sizes and silently ignores the further symbols without error.

```
1.0 ...KB
1.0 42.0 B
```
This change makes those illegal.

`1 000 B` was also subject to the bad `skip_while` ignoring the
following `000`. In this version of the fix whitespace is not accepted
as a digit separator. So it will raise an error if the user doesn't
explicitly strip the whitespace instead of reporting a wrong value.

Author: sholderbach

src/parse.rs

@@ -10,9 +10,7 @@ impl std::str::FromStr for ByteSize {
         let number = take_while(value, |c| c.is_ascii_digit() || c == '.');
         match number.parse::<f64>() {
             Ok(v) => {
-                let suffix = skip_while(value, |c| {
-                    c.is_whitespace() || c.is_ascii_digit() || c == '.'
-                });
+                let suffix = skip_while(&value[number.len()..], char::is_whitespace);
                 match suffix.parse::<Unit>() {
                     Ok(u) => Ok(Self((v * u) as u64)),
                     Err(error) => Err(format!(