Fix sly-goto-xref bug and refactor source-navigation functions

When re-accessing the *xref* buffer, `sly-goto-xref' didn't really
switch to the source. The remaining refactoring is in preparation of a
more serious uniformization of source-finding behaviour across all SLY
buffers.

* sly.el (sly--display-source-location)
(sly--pop-to-source-location): New from
`sly-pop-to-location'. Always highlights, new `delete-current'
method.
(sly--move-to-source-location): Renamed from
`sly-goto-source-location'.
(sly--display-source-location): New from
`sly-show-source-location'.
(sly-show-source-location, sly-pop-to-location): Deleted.
(sly-recompile-location, sly-recompile-locations)
(sly-canonicalized-location): Use new
`sly--move-to-source-location'.
(sly-edit-uses, sly-edit-definition-cont, sly-goto-xref)
(sly-goto-next-xref): Use `sly--pop-to-source-location'
(sly-xref-show-location, sly-inspector-show-source)
(sly-show-xref): Use `sly--display-source-location'
(sly-xref-mode-map): Delete `next-line' and `previous-line'
remappings.