perf: Speed up detection of text files

We reduce how much of the buffer we walk twice which should speed up
large files.  We still load the entire file into memory which will still
hurt binary files.

This is part of #34.

typos/src/checks.rs

@@ -121,7 +121,8 @@ impl<'d, 'p> Checks<'d, 'p> {
         }
 
         let buffer = std::fs::read(path)?;
-        if !explicit && !self.binary && buffer.find_byte(b'\0').is_some() {
+        let null_max = std::cmp::min(buffer.len(), 1024);
+        if !explicit && !self.binary && buffer[0..null_max].find_byte(b'\0').is_some() {
             let msg = report::BinaryFile {
                 path,
                 non_exhaustive: (),