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/struct/name_order.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test the that field order doesn't matter for structs.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: {.x: i32, .y: i32} = {.y = 2, .x = 3};
   return 0;
 }