Add lf em 4x70 autorecover

Also:
* common.h: Increase safety of some of the macros.
  Parenthesizing the macro parameters ... a best practice.
* firmware: partially-tracked source of "extra bits" messages.
  Add a TODO comment for further study.
* Improve reliability of `lf em 4x70 writekey`
  Authenticate w/new key after it is written.
  Particularly important for glass modules,
  or other tags with weaker coupling.

include/em4x70.h

@@ -39,6 +39,7 @@ typedef struct {
 
     // Used for writing address
     uint8_t address;
+    // ISSUE: Presumes target is little-endian
     uint16_t word;
 
     // PIN to unlock
@@ -53,6 +54,7 @@ typedef struct {
     uint8_t crypt_key[12];
 
     // used for bruteforce the partial key
+    // ISSUE: Presumes target is little-endian
     uint16_t start_key;
 
 } em4x70_data_t;