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/experimental_continuation/await_maintains_scope.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test access to block-scoped variables upon resuming a continuation.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var y: i32 = 0;
   __continuation k {
     var x: i32 = 0;

executable_semantics/testdata/experimental_continuation/creation_is_noop.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test that creating a continuation doesn't do anything.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = 0;
   __continuation k {
     x = x + 1;

executable_semantics/testdata/experimental_continuation/fail_continuation_syntax.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = 0;
   __continuation k x = 3;
   __run k;

executable_semantics/testdata/experimental_continuation/fail_lifetime.carbon

@@ -23,7 +23,7 @@ fn capture() -> __Continuation {
   return k;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var k: __Continuation = capture();
   __run k; // error, lifetime of x is over
   return 0;

executable_semantics/testdata/experimental_continuation/recursive.carbon

@@ -27,7 +27,7 @@ fn CountUpTo(x: i32) -> i32 {
   }
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   __continuation k {
     CountUpTo(5);
   }

executable_semantics/testdata/experimental_continuation/run.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test creating and running a continuation.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = 0;
   __continuation k {
     x = x + 1;

executable_semantics/testdata/experimental_continuation/run_with_await.carbon

@@ -14,7 +14,7 @@ package ExecutableSemanticsTest api;
 // Test pausing a continuation with `__await` and restarting it with
 // `__run`.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = 0;
   __continuation k {
     x = x + 1;

executable_semantics/testdata/experimental_continuation/shallow_copy.carbon

@@ -22,7 +22,7 @@ fn Foo() {
   x = x + 2;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   __continuation k1 {
     Foo();
   }