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/string/basic.carbon

@@ -18,6 +18,6 @@ fn CompareStr(s: String) -> i32 {
   return 1;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return CompareStr("str");
 }

executable_semantics/testdata/string/fail_hex_lower.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   Print("str\xaa");
   return 0;
 }

executable_semantics/testdata/string/fail_hex_truncated.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   Print("str\x");
   return 0;
 }

executable_semantics/testdata/string/fail_invalid_escape.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   Print("str\e");
   return 0;
 }

executable_semantics/testdata/string/fail_newline.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   Print("new
 line");
   return 0;

executable_semantics/testdata/string/fail_octal.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   Print("str\01");
   return 0;
 }

executable_semantics/testdata/string/fail_tab.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   Print("new	line");
   return 0;
 }

executable_semantics/testdata/string/hex.carbon

@@ -18,6 +18,6 @@ fn CompareStr(s: String) -> i32 {
   return 1;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return CompareStr("\x73\x74\x72\x3B");
 }

executable_semantics/testdata/string/newline.carbon

@@ -18,6 +18,6 @@ fn CompareStr(s: String) -> i32 {
   return 1;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return CompareStr("str\n");
 }