descriptor: introduce a method to get the satisfaction size

In the wallet code, we are currently estimating the size of a signed
input by doing a dry run of the signing logic. This is unnecessary as
all outputs we are able to sign for can be represented by a descriptor,
and we can derive the size of a satisfaction ("signature") from the
descriptor itself directly.
In addition, this approach does not scale: getting the size of a
satisfaction through a dry run of the signing logic is only possible for
the most basic scripts.

This commit introduces the computation of the size of satisfaction per
descriptor. It's a bit intricate for 2 main reasons:
- We want to conserve the behaviour of the current dry-run logic used by
  the wallet that sometimes assumes ECDSA signatures will be low-r,
  sometimes not (when we don't create them).
- We need to account for the witness discount. A single descriptor may
  sometimes benefit of it, sometimes not (for instance `pk()` if used as
  top-level versus if used inside `wsh()`).

src/test/fuzz/descriptor_parse.cpp

@@ -112,7 +112,7 @@ static void TestDescriptor(const Descriptor& desc, FlatSigningProvider& sig_prov
 {
     // Trivial helpers.
     (void)desc.IsRange();
-    (void)desc.IsSolvable();
+    const bool is_solvable{desc.IsSolvable()};
     (void)desc.IsSingleType();
     (void)desc.GetOutputType();
 
@@ -131,7 +131,18 @@ static void TestDescriptor(const Descriptor& desc, FlatSigningProvider& sig_prov
     // If we could serialize to script we must be able to infer using the same provider.
     if (!out_scripts.empty()) {
         assert(InferDescriptor(out_scripts.back(), sig_provider));
+
+        // The ScriptSize() must match the size of the serialized Script. (ScriptSize() is set for all descs but 'combo()'.)
+        const bool is_combo{!desc.IsSingleType()};
+        assert(is_combo || desc.ScriptSize() == out_scripts.back().size());
     }
+
+    const auto max_sat_maxsig{desc.MaxSatisfactionWeight(true)};
+    const auto max_sat_nonmaxsig{desc.MaxSatisfactionWeight(true)};
+    // We must be able to estimate the max satisfaction size for any solvable descriptor (but combo).
+    const bool is_nontop_or_nonsolvable{!is_solvable || !desc.GetOutputType()};
+    const bool is_input_size_info_set{max_sat_maxsig && max_sat_nonmaxsig};
+    assert(is_input_size_info_set || is_nontop_or_nonsolvable);
 }
 
 void initialize_descriptor_parse()