Add geiser-eval-buffer & geiser-eval-buffer-and-go

These functions are similar to geiser-eval-region and
geiser-eval-region-and-go, however they allow the user to operate on the
entire buffer, not requiring the user to narrow to a specific region.
This also differs slightly from geiser-compile-current-buffer as
geiser-eval-buffer does not require the contents of the buffer to
be saved prior to being sent to the REPL.  Documentaion has also been
updated to include references to the new methods and their keybindings.