Speed up sum by using reasonable read buffer sizes.

Use a 4K read buffer for each of the checksum functions, which seems
reasonable. This improves the performance of BSD checksums on
odyssey1024.txt from 399ms to 325ms on my laptop, and of SysV
checksums from 242ms to 67ms.

src/uu/sum/src/sum.rs

@@ -24,7 +24,7 @@ static SUMMARY: &str = "Checksum and count the blocks in a file.\n\
                         With no FILE, or when  FILE is -, read standard input.";
 
 fn bsd_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
-    let mut buf = [0; 1024];
+    let mut buf = [0; 4096];
     let mut blocks_read = 0;
     let mut checksum: u16 = 0;
     loop {
@@ -44,7 +44,7 @@ fn bsd_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
 }
 
 fn sysv_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
-    let mut buf = [0; 512];
+    let mut buf = [0; 4096];
     let mut blocks_read = 0;
     let mut ret = 0u32;