:sig
u64 to_u64(String s, u64 fallback = 0)

:params
s : string to parse
fallback : value returned when `s` is empty or not a complete unsigned integer
return value : parsed u64, or `fallback`.

:see
>types
to_s64
to_f64
to_bool
int_val

:content
Parses a trimmed string as a base-10 unsigned integer. An optional leading `+`
is accepted. Negative and overflowing values return the fallback.

Unlike `int_val()`, the whole trimmed string must be consumed by the parser. Empty strings and partially parsed values such as `"42px"` return the fallback.

:example
print(to_u64("7"), "\n");
