Merge bitcoin#7: Declare single-argument (non-converting) constructor…

…s "explicit"

fe805ea Declare single-argument (non-converting) constructors "explicit" (practicalswift)

Pull request description:

  Rebased on bitcoin#4.

  Closes bitcoin#6, which stripped the original author.

Tree-SHA512: 4f4164fb1bbce54e5e28a5cf1327c4d7a2058e74384d1173de843acbfb71a3dc4a11e559650b86c6ac9063ea9c76c96fa371bff985d794c8dc904448f4ecd095

lib/univalue_utffilter.h

@@ -13,7 +13,7 @@
 class JSONUTF8StringFilter
 {
 public:
-    JSONUTF8StringFilter(std::string &s):
+    explicit JSONUTF8StringFilter(std::string &s):
         str(s), is_valid(true), codepoint(0), state(0), surpair(0)
     {
     }