Rework struct toString()

[Kronos3]

This code cleans up the toString() on structs which as a side benefit reduces the stack memory required to format the struct into a string (Closes #722).

Before
Format each individual member into it's own string on the stack and then use a big old Fw::String.format() call with a predefined format string to glob all the members into a single string.

Now
Move through each member in the string and append the formatted member to the string. There is optionally a single Fw::String tmp which members will use to format into if needed before appending to the output.

• Test with FppTest
• Test with check-cpp

[bocchino]

Looks great! The new code gen strategy is much better.

I confirmed that the fpp-check tests pass and the FppTest tests pass in nasa/fprime/devel.