Fix inconsistend use of the byte string phrase.

I have changed all instances of `bytestring` into `byte string`. The latter form seems to be the proper one.

Language documentation also use `byte slice` naming, but I'm not sure it's a reference to the same thing, so I'm open to feedback from the community.

Author: tomasz-rozanski

src/std/str.md

@@ -120,19 +120,19 @@ use std::str;
 
 fn main() {
     // Note that this is not actually a `&str`
-    let bytestring: &[u8; 20] = b"this is a bytestring";
+    let bytestring: &[u8; 20] = b"this is a byte string";
 
     // Byte arrays don't have the `Display` trait, so printing them is a bit limited
-    println!("A bytestring: {:?}", bytestring);
+    println!("A byte string: {:?}", bytestring);
 
-    // Bytestrings can have byte escapes...
+    // Byte strings can have byte escapes...
     let escaped = b"\x52\x75\x73\x74 as bytes";
     // ...but no unicode escapes
     // let escaped = b"\u{211D} is not allowed";
     println!("Some escaped bytes: {:?}", escaped);
 
 
-    // Raw bytestrings work just like raw strings
+    // Raw byte strings work just like raw strings
     let raw_bytestring = br"\u{211D} is not escaped here";
     println!("{:?}", raw_bytestring);
 
@@ -144,7 +144,7 @@ fn main() {
     let quotes = br#"You can also use "fancier" formatting, \
                     like with normal raw strings"#;
 
-    // Bytestrings don't have to be UTF-8
+    // Byte strings don't have to be UTF-8
     let shift_jis = b"\x82\xe6\x82\xa8\x82\xb1\x82"; // "ようこそ" in SHIFT-JIS
 
     // But then they can't always be converted to `str`