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/assignment_copy/destruct_original.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test that assignment performs a copy and does not create an alias.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = -1;
   {
      var y: i32 = 0;

executable_semantics/testdata/assignment_copy/reassign_original.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test that assignment performs a copy and does not create an alias.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = 0;
   var y: i32 = x;
   x = 1;