Further reduce the runtime of parse_u64_digits

Here instead of figuring out the extents of the integer ahead of time we
check for the spaces while we compute the number itself. This further
reduces the runtime of the beforementioned case (see previous commit) to:

    580.57 msec     task-clock:u              #    0.997 CPUs utilized
 1,843,957,595      cycles:u                  #    3.176 GHz
 5,901,570,558      instructions:u            #    3.20  insn per cycle

`perf report` still shows that the most of the time is spent parsing
sysv archive names (which makes sense – its pretty much all the program
does after all!).

src/read/archive.rs

@@ -267,17 +267,19 @@ impl<'data> ArchiveMember<'data> {
 
 // Ignores bytes starting from the first space.
 fn parse_u64_digits(digits: &[u8], radix: u32) -> Option<u64> {
-    let len = memchr::memchr(b' ', digits).unwrap_or(digits.len());
-    let digits = &digits[..len];
-    if digits.is_empty() {
+    if let [b' ', ..] = digits {
         return None;
     }
     let mut result: u64 = 0;
     for &c in digits {
-        let x = (c as char).to_digit(radix)?;
-        result = result
-            .checked_mul(u64::from(radix))?
-            .checked_add(u64::from(x))?;
+        if c == b' ' {
+            return Some(result);
+        } else {
+            let x = (c as char).to_digit(radix)?;
+            result = result
+                .checked_mul(u64::from(radix))?
+                .checked_add(u64::from(x))?;
+        }
     }
     Some(result)
 }
@@ -286,7 +288,7 @@ fn parse_sysv_extended_name<'data>(digits: &[u8], names: &'data [u8]) -> Result<
     let offset = parse_u64_digits(digits, 10).ok_or(())?;
     let offset = offset.try_into().map_err(|_| ())?;
     let name_data = names.get(offset..).ok_or(())?;
-    let name = match memchr::memchr2(b'/', 0, name_data) {
+    let name = match memchr::memchr2(b'/', b'\0', name_data) {
         Some(len) => &name_data[..len],
         None => name_data,
     };
@@ -303,7 +305,7 @@ fn parse_bsd_extended_name<'data, R: ReadRef<'data>>(
     let len = parse_u64_digits(digits, 10).ok_or(())?;
     *size = size.checked_sub(len).ok_or(())?;
     let name_data = data.read_bytes(offset, len)?;
-    let name = match memchr::memchr(0, name_data) {
+    let name = match memchr::memchr(b'\0', name_data) {
         Some(len) => &name_data[..len],
         None => name_data,
     };