all: more cleanups

Changes include:
  - extract headers.h from conf.h
  - use "byte" in some more places
  - unify "const" coding style
  - pefile.cpp: make sort order deterministic
  - simplify some other pefile.cpp code
  - CI updates

src/p_tmt.h

@@ -65,11 +65,11 @@ protected:
     int big_relocs = 0;
 
     struct alignas(1) tmt_header_t {
-        char _[16]; // signature,linkerversion,minversion,exesize,imagestart
+        byte _[16]; // signature,linkerversion,minversion,exesize,imagestart
         LE32 imagesize;
-        char __[4]; // initial memory
+        byte __[4]; // initial memory
         LE32 entry;
-        char ___[12]; // esp,numfixups,flags
+        byte ___[12]; // esp,numfixups,flags
         LE32 relocsize;
     };
     tmt_header_t ih, oh;