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/function/multiple_args.carbon

@@ -16,6 +16,6 @@ fn f(x: i32, y: i32) -> i32 {
   return x + y;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return f(2,3) - 5;
 }