bytes: clean up and simplify encoding of HexBytes (#6810)

As written, the encoding step unnecessarily made and moved multiple copies of
the encoded representation. Reduce this to a single allocation and encode the
data in-place so that a shift is no longer required.

Also: Add a test to ensure letter digits are capitalized, which was previously not
verified but was expected downstream.

No functional changes.

libs/bytes/bytes_test.go

@@ -37,6 +37,7 @@ func TestJSONMarshal(t *testing.T) {
 		{[]byte(``), `{"B1":"","B2":""}`},
 		{[]byte(`a`), `{"B1":"YQ==","B2":"61"}`},
 		{[]byte(`abc`), `{"B1":"YWJj","B2":"616263"}`},
+		{[]byte("\x1a\x2b\x3c"), `{"B1":"Gis8","B2":"1A2B3C"}`},
 	}
 
 	for i, tc := range cases {