improve `--heap-size-hint` arg handling (#48050)
Previously, `--heap-size-hint` would silently ignore many flavors of
"bad" input, parsing things like "3PB" as 3 bytes. This change makes it
significantly less permissive, erroring unless it can parse a number
(still relying on the C `sscanf` `%Lf` format specifier there) with an
optional unit (case-insensitive, either with or without the trailing
`b`). Also test it.
