Rename main to Main for Carbon style consistency (#939)

Main() is more consistency with Carbon's naming guidelines. C# offers some precedent: https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/program-structure/main-command-line

executable_semantics/testdata/string/basic.carbon

@@ -18,6 +18,6 @@ fn CompareStr(s: String) -> i32 {
   return 1;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return CompareStr("str");
 }