This commit fixes compiler bug where it fails when optional values are
compiled into absent values in nested calls.
- Throw exception with more context for easier future debugging.
- Add better validation of argument values for nested calls.
- Refactor `FunctionCallCompiler` for better clarity and modularize it
to make it more maintainable and testable.
- Refactor related interface to not have `I` prefix, and
function/variable names for better clarity.
Context:
Discovered this issue while attempting to call
`RunInlineCodeAsTrustedInstaller` which in turn invokes `RunPowerShell`
for issue #246. This led to the realization that despite parameters
flagged as optional, the nested argument compilation didn't support
them.
