Replace assignments to StringBase& in generated code

[bocchino]

Instead of sb = "foo" we should do sb.format("%s", "foo"). In some cases this will remove an extra buffer and an snprintf. Then we can make the copy assignment operator protected in the StringBase class.

[timcanham]

I've noticed during past projects that the format specifier strings can take up a lot of code, and the assignment operator is a natural way to code. Could we move the operator to the derived class?

[bocchino]

In most cases the proposed change will reduce code. For example instead of doing

• Declare string buffer
• snprintf into string buffer
• Assign string buffer into string base object

we'll just do

• Format into string base object

In a few cases we'll have to add a format string "%s", but this is a negligible difference, and I think it only happens in unit tests. We could keep sb = "foo" in those cases, but then I think we'll need to have a public assignment operator in the base class, since the base class appears on the left-hand side of the assignment.

[bocchino]

Covered by #423. We can keep the assignment into StringBase for now, but we should remove the extra buffer copy where possible.