better naming of Record::Mutator content moving operation

while the original name, 'replace', conveys the intention,
this more standard name 'swap' reveals what is done
and thus opens a wider array of possible usage

src/lib/diff/record.hpp

@@ -414,7 +414,7 @@ namespace diff{
         }
       
       void
-      replace (Rec& existingInstance)  noexcept
+      swap (Rec& existingInstance)  noexcept
         {
           std::swap (existingInstance, record_);
         }

tests/library/diff/generic-record-representation-test.cpp

@@ -330,7 +330,7 @@ namespace test{
           
           CHECK (mut != aa);
           
-          mut.replace(a);
+          mut.swap (a);
           CHECK (isnil (mut));
           CHECK (Seq({"a = α", "b = β", "⟂", "a"}) == contents(a));
           CHECK (Seq({"a = 1", "a"}) == contents(aa));

tests/library/diff/generic-record-update-test.cpp

@@ -158,7 +158,7 @@ namespace test{
           CHECK ( isnil (content));
           CHECK (!isnil (mut));
           
-          mut.replace(subject);
+          mut.swap (subject);
           CHECK (Seq({"a = α", "b = β", "γ", "δ", "ε"}) == contents(subject));
           CHECK ("Rec(🌰| a = α, b = β |{γ, δ, ε})" == string(subject));
         }