Merge feee029d29 into merged_master (Elements PR ElementsProject/elements#1020)

Messy merge conflicts because this PR backported some ad-hoc stuff from
upstream while keeping a few things that upstream deleted. Hopefully
reviewing is easier than doing this in the first place, since ultimately
all I did was delete code from one side or another of the conflicts.
(Ok, I also changed some boost optional stuff to std::optional, and had
to patch up a test file for test framework changes.)

When reviewing the detailed crypto, bear in mind that taptweaks, like all
hashes are the kind of crypto that cannot be subtly wrong -- it will either
fail very hard or be correct. And we have independent implementations in
C++ and Python that cross-check each other, si it's unlikely to be the former.

src/script/script_error.h

@@ -86,10 +86,20 @@ typedef enum ScriptError_t
 
     // ELEMENTS:
     SCRIPT_ERR_RANGEPROOF,
-    SCRIPT_ERR_PEDERSEN_TALLY
+    SCRIPT_ERR_PEDERSEN_TALLY,
+
+
+    /* Elements: New tapscript related errors */
+    SCRIPT_ERR_SHA2_CONTEXT_LOAD,
+    SCRIPT_ERR_SHA2_CONTEXT_WRITE,
+    SCRIPT_ERR_INTROSPECT_CONTEXT_UNAVAILABLE,
+    SCRIPT_ERR_INTROSPECT_INDEX_OUT_OF_BOUNDS,
+    SCRIPT_ERR_EXPECTED_8BYTES,
+    SCRIPT_ERR_ARITHMETIC64,
+    SCRIPT_ERR_ECMULTVERIFYFAIL
 } ScriptError;
 
-#define SCRIPT_ERR_LAST SCRIPT_ERR_ERROR_COUNT
+#define SCRIPT_ERR_LAST SCRIPT_ERR_ECMULTVERIFYFAIL
 
 std::string ScriptErrorString(const ScriptError error);