GcodeDisplay: Use iterator instead of indexing

This avoids allocations due to _M_leak since possible_command is
mutable.
Gcode: Make command const

This also kills allocations from indexing
SimpleShell: Reserve buffer before appending
Player: Pop Gcode before deleting
StreamOutput: Buffer allocated by malloc being freed with delete

Thanks to adamgr for noticing this.
StreamOutput: Kill dead code
Merge pull request #100 from bgamari/fixes

Various things encountered during search for source of heap corruption