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/tuple/match_nested.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test matching of a tuple inside a tuple.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = ((1,2),(3,4));
   match (t) {
     case ((a: auto, b: auto), c: auto) =>