Added optional length to caller() method and support for Range argument
Minor whitespace fixes
Fixed off-by-one error and handled malformed arguments
Tweaks to improved 2.0 caller/backtrace logic.

* Reuse level/length logic across methods
* Reuse gathering logic across methods
* Handle special cases (length = 0, size < 0) properly
* Other tweaks to get 2.0 test_backtrace 100%
* Style fixes