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/fnty.carbon

@@ -15,7 +15,7 @@ fn add1(x: i32) -> i32 {
   return x + 1;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var f: __Fn(i32)->i32 = add1;
   return f(-1);
 }