Add indirection to allow for multiple inspectors.

Inspectors are identified by symbol, so from emacs the default is called
DEFAULT and additional ones can be used using the prefix argument to
each interactive function (when applicable).

Is still missing some functions, basics work though.

* slime.el (slime-buffer-name): Additional `postfix` argument.
(slime-inspect, slime-inspector-buffer, slime-inspector-pop,
slime-inspector-next, slime-inspector-quit, slime-inspector-describe,
slime-inspector-eval, slime-inspector-history, slime-inspector-reinspect,
slime-inspector-toggle-verbose): Additional `inspector-name` argument, which is
set by `interactive`.
(slime-open-inspector, slime-inspector-operate-on-point,
slime-inspector-pprint, slime-inspector-show-source, slime-inspector-fetch):
Pass the `inspector-name` to the SWANK side as a symbol.

* swank.lisp (value-spec-ref): `:inspector` ref gets a `name` argument which
refers to the inspector in question.
(reset-inspector, init-inspector, emacs-inspect/istate, istate>elisp,
inspector-nth-part, inspect-nth-part, inspector-range,
inspector-call-nth-action, inspector-pop, inspector-next, inspector-reinspect,
inspector-toggle-verbose, inspector-eval, inspector-history, quit-inspector,
describe-inspectee, pprint-inspector-part, inspect-in-frame,
inspect-current-condition, inspect-frame-var): Additional `name` argument like
above.
(ensure-istate-metadata, print-part-to-string): Refer to the inspector state
indirectly by `*inspector-name*`.
(inspect-object): Inspector instance passed as additional argument.

* contrib/slime-repl.el (slime-repl-inspect): Additional `inspector-name`
argument to refer to the inspector in question.

* contrib/slime-fancy-inspector.el (slime-inspector-copy-down-to-repl): Pass
the `slime-inspector-name` to the SWANK side as a symbol.