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/generic_function/fail_not_addable.carbon

@@ -17,6 +17,6 @@ fn id[T:! Type](x: T) -> T {
   return x + 0;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return id(0);
 }