fuzz: [refactor] Use std::span over FuzzBufferType in descriptor utils

They are exactly the same, but the descriptor utils should not prescribe
to use the FuzzBufferType. Using a dedicated type for them clarifies
that the utils are not tied to FuzzBufferType.

Also, while touching the lines, use `const` only where it is meaningful.

src/test/fuzz/util/descriptor.cpp

@@ -74,7 +74,7 @@ std::optional<std::string> MockedDescriptorConverter::GetDescriptor(std::string_
     return desc;
 }
 
-bool HasDeepDerivPath(const FuzzBufferType& buff, const int max_depth)
+bool HasDeepDerivPath(std::span<const uint8_t> buff, const int max_depth)
 {
     auto depth{0};
     for (const auto& ch: buff) {
@@ -88,7 +88,7 @@ bool HasDeepDerivPath(const FuzzBufferType& buff, const int max_depth)
     return false;
 }
 
-bool HasTooManySubFrag(const FuzzBufferType& buff, const int max_subs, const size_t max_nested_subs)
+bool HasTooManySubFrag(std::span<const uint8_t> buff, const int max_subs, const size_t max_nested_subs)
 {
     // We use a stack because there may be many nested sub-frags.
     std::stack<int> counts;
@@ -112,7 +112,7 @@ bool HasTooManySubFrag(const FuzzBufferType& buff, const int max_subs, const siz
     return false;
 }
 
-bool HasTooManyWrappers(const FuzzBufferType& buff, const int max_wrappers)
+bool HasTooManyWrappers(std::span<const uint8_t> buff, const int max_wrappers)
 {
     // The number of nested wrappers. Nested wrappers are always characters which follow each other so we don't have to
     // use a stack as we do above when counting the number of sub-fragments.

src/test/fuzz/util/descriptor.h

@@ -53,7 +53,7 @@ constexpr int MAX_DEPTH{2};
  * Whether the buffer, if it represents a valid descriptor, contains a derivation path deeper than
  * a given maximum depth. Note this may also be hit for deriv paths in origins.
  */
-bool HasDeepDerivPath(const FuzzBufferType& buff, const int max_depth = MAX_DEPTH);
+bool HasDeepDerivPath(std::span<const uint8_t> buff, int max_depth = MAX_DEPTH);
 
 //! Default maximum number of sub-fragments.
 constexpr int MAX_SUBS{1'000};
@@ -64,8 +64,8 @@ constexpr size_t MAX_NESTED_SUBS{10'000};
  * Whether the buffer, if it represents a valid descriptor, contains a fragment with more
  * sub-fragments than the given maximum.
  */
-bool HasTooManySubFrag(const FuzzBufferType& buff, const int max_subs = MAX_SUBS,
+bool HasTooManySubFrag(std::span<const uint8_t> buff, int max_subs = MAX_SUBS,
-                       const size_t max_nested_subs = MAX_NESTED_SUBS);
+                       size_t max_nested_subs = MAX_NESTED_SUBS);
 
 //! Default maximum number of wrappers per fragment.
 constexpr int MAX_WRAPPERS{100};
@@ -74,6 +74,6 @@ constexpr int MAX_WRAPPERS{100};
  * Whether the buffer, if it represents a valid descriptor, contains a fragment with more
  * wrappers than the given maximum.
  */
-bool HasTooManyWrappers(const FuzzBufferType& buff, const int max_wrappers = MAX_WRAPPERS);
+bool HasTooManyWrappers(std::span<const uint8_t> buff, int max_wrappers = MAX_WRAPPERS);
 
 #endif // BITCOIN_TEST_FUZZ_UTIL_DESCRIPTOR_H