Improve macro hygiene

This commit improves hygiene and formatting of macro definitions
throughout the library. Specifically:
- It adds brackets around parameters to avoid unintended
  interpretation of arguments, e.g. due to operator precedence.
- It adds uses of the `do { ... } while( 0 )` idiom for macros that
  can be used as commands.

include/mbedtls/asn1write.h

@@ -33,11 +33,12 @@
 #include "asn1.h"
 
 #define MBEDTLS_ASN1_CHK_ADD(g, f)                      \
-    do {                                                \
-        if( ( ret = f ) < 0 )                           \
+    do                                                  \
+    {                                                   \
+        if( ( ret = (f) ) < 0 )                         \
             return( ret );                              \
         else                                            \
-            g += ret;                                   \
+            (g) += ret;                                 \
     } while( 0 )
 
 #ifdef __cplusplus