Pass std::string by value where applicable.

By taking the std::string by value in the constructor, this allows for
certain situations where copies can be elided entirely (when moving an
instance into the constructor)

e.g.

std::string var = ...

...

... = LiteralString(std::move(var)) // Or whatever other initialization
                                    // is done.

No copy will be done in this case, the move transfers it into the
constructor, and then the move within the initializer list transfers it
into the member variable.

tl;dr: This allows the calling code to potentially construct less
std::string instances by allowing moving into the parameters themselves.

src/literal_string.cpp

@@ -10,7 +10,7 @@
 
 namespace Sirit {
 
-LiteralString::LiteralString(const std::string& string) : string(string) {
+LiteralString::LiteralString(std::string string) : string{std::move(string)} {
     operand_type = OperandType::String;
 }