pry/pry-stack_explorer: Walk the stack in a Pry session

Pry::StackExplorer is a plugin for Pry that allows navigating the call stack. From the point a Pry session is started, the user can move up the stack through parent frames, examine state, and even evaluate code. Unlike ruby-debug, pry-stack_explorer incurs no runtime cost and enables navigation right up the call-stack to the birth of the program. The up, down, frame and stack commands are provided. See Pry's in-session help for more information on any of these commands.