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/param_lifetime.carbon

@@ -19,7 +19,7 @@ fn f(x: i32) -> i32 {
   return 0;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var a: i32 = 0; var b: i32 = 1;
   f(a);
   b = a;