Revise code generation for parameters

[bocchino]

• Make the behavior of external parameters consistent with the behavior of internal parameters. In particular, respect the default values provided for external parameters.
• Move the scratch parameter buffer from the stack to a component instance member, to avoid overflowing the stack.
• Clean up the generated code.

Closes #984.

Tandem with https://github.com/bocchino/fprime/tree/fpp-issue-984-extern-param-defaults.

Implementation Note

In the old code for loading external parameters, if there was no valid value to send to the external parameter interface, the interface was never called. In the new code, the interface is called with an INVALID flag argument. The new behavior provides more information to the external interface. However, the external interface will need to respect the rule that if it is called with an INVALID flag argument, then the data buffer does not contain valid data.

Review Notes

The diff of the C++ output for the component base classes may be hard to review. Here is a simple model that illustrates the new code generation strategy:

module Fw {
  port Time
  port PrmGet
  port PrmSet
  port Cmd
  port CmdResponse
  port CmdReg
}

passive component C {

  time get port timeGetOut

  param get port paramGet

  param set port paramSet

  command recv port cmdIn

  command resp port cmdResponseOut

  command reg port cmdRegIn

  param InternalU32Default: U32 default 42

  external param ExternalU32Default: U32 default 42

  param InternalU32NoDefault: U32

  external param ExternalU32NoDefault: U32

}

You can run this model through fpp-to-cpp to see the four cases: internal parameter with default, external parameter with default, internal parameter with no default, and external parameter with no default.

[Kronos3]

I think this codegen looks good though we may want to revisit the use of the scratch buffer instead of stack buffer in the future.