Updated literal suffixes to isize and usize.

The literals 0is and 0us have been deprecated in favor of 0isize and 0usize.
Reference: rust-lang/rfcs#573

syntax/rust.vim

@@ -137,10 +137,10 @@ syn region    rustAttribute   start="#!\?\[" end="\]" contains=rustString,rustDe
 syn region    rustDerive      start="derive(" end=")" contained contains=rustTrait
 
 " Number literals
-syn match     rustDecNumber   display "\<[0-9][0-9_]*\%([iu]\%(s\|8\|16\|32\|64\)\)\="
-syn match     rustHexNumber   display "\<0x[a-fA-F0-9_]\+\%([iu]\%(s\|8\|16\|32\|64\)\)\="
-syn match     rustOctNumber   display "\<0o[0-7_]\+\%([iu]\%(s\|8\|16\|32\|64\)\)\="
-syn match     rustBinNumber   display "\<0b[01_]\+\%([iu]\%(s\|8\|16\|32\|64\)\)\="
+syn match     rustDecNumber   display "\<[0-9][0-9_]*\%([iu]\%(size\|8\|16\|32\|64\)\)\="
+syn match     rustHexNumber   display "\<0x[a-fA-F0-9_]\+\%([iu]\%(size\|8\|16\|32\|64\)\)\="
+syn match     rustOctNumber   display "\<0o[0-7_]\+\%([iu]\%(size\|8\|16\|32\|64\)\)\="
+syn match     rustBinNumber   display "\<0b[01_]\+\%([iu]\%(size\|8\|16\|32\|64\)\)\="
 
 " Special case for numbers of the form "1." which are float literals, unless followed by
 " an identifier, which makes them integer literals with a method call or field access,