Deprecate use of unary '+'

The unary '+' serves no meaningful purpose in Solidity and it makes it
possible to produce typos with dagerous implications (e.g. 'a =+5 '),
so we are deprecating it.  The SyntaxChecker currently issues warnings
on the unary '+' but will still compile it for now.

libsolidity/analysis/SyntaxChecker.h

@@ -32,6 +32,7 @@ namespace solidity
  * The module that performs syntax analysis on the AST:
  *  - whether continue/break is in a for/while loop.
  *  - whether a modifier contains at least one '_'
+ *  - issues deprecation warnings for unary '+'
  */
 class SyntaxChecker: private ASTConstVisitor
 {
@@ -43,6 +44,7 @@ public:
 
 private:
 	/// Adds a new error to the list of errors.
+	void warning(SourceLocation const& _location, std::string const& _description);
 	void syntaxError(SourceLocation const& _location, std::string const& _description);
 
 	virtual bool visit(SourceUnit const& _sourceUnit) override;
@@ -60,6 +62,8 @@ private:
 	virtual bool visit(Continue const& _continueStatement) override;
 	virtual bool visit(Break const& _breakStatement) override;
 
+	virtual bool visit(UnaryOperation const& _operation) override;
+
 	virtual bool visit(PlaceholderStatement const& _placeholderStatement) override;
 
 	ErrorList& m_errors;