thread context scratch system; converge on transition idiom in sample code

tests/cpp_build_test.cpp

@@ -1,9 +1,14 @@
 #include "md.h"
 #include "md.c"
 
+static MD_Arena *arena = 0;
+
 int main(void)
 {
-    MD_Arena *arena = MD_ArenaNew(1ull << 40);
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
+    arena = MD_ArenaNew(1ull << 40);
     
     printf("%d\n", MD_CPP_VERSION);
     

tests/sanity_tests.c

@@ -120,6 +120,9 @@ TokenMatch(MD_Token token, MD_String8 string, MD_TokenKind kind)
 
 int main(void)
 {
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
     arena = MD_ArenaNew(1ull << 40);
     
     Test("Lexer")

tests/unicode_test.c

@@ -16,6 +16,9 @@ void run_test_on_string(MD_String8 string)
 
 int main(void)
 {
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
     arena = MD_ArenaNew(1ull << 40);
     
     // TODO(allen): throw more at this.