Clean up crc module

Use better names, remove quite a bit of dead code.
Change `><` to a name that's actually descriptive.

compiler/extccomp.nim

@@ -572,13 +572,13 @@ proc getCompileCFileCmd*(cfilename: string, isExternal = false): string =
     "nim", quoteShell(getPrefixDir()),
     "lib", quoteShell(libpath)])
 
-proc footprint(filename: string): TCrc32 =
-  # note, '><' further modifies a crc value with a string.
-  result = crcFromFile(filename) ><
-      platform.OS[targetOS].name ><
-      platform.CPU[targetCPU].name ><
-      extccomp.CC[extccomp.cCompiler].name ><
-      getCompileCFileCmd(filename, true)
+proc footprint(filename: string): SecureHash =
+  result = secureHash(
+    $secureHashFile(filename) &
+    platform.OS[targetOS].name &
+    platform.CPU[targetCPU].name &
+    extccomp.CC[extccomp.cCompiler].name &
+    getCompileCFileCmd(filename, true))
 
 proc externalFileChanged(filename: string): bool =
   if gCmd notin {cmdCompileToC, cmdCompileToCpp, cmdCompileToOC, cmdCompileToLLVM}: