With evtclib 0.2, every boss has at least one name without space, so
every boss can be used. Still, for completeness's and consistency's
sake, we want to allow users to also specify boss names with spaces in
them. For example, if we print "Qadim the Peerless" as the name of the
boss, we might expect
    raidgrep -- -boss "Qadim the Peerless"
to work (instead of -boss qadimp). Therefore, we now allow boss names to
be quoted, so that we can properly persist the whitespace.

Having a ::new on each of the filter types was a bit weird, especially
because we returned Box<dyn ...> instead of Self (and clippy rightfully
complained). With this patch, we now have a bunch of normal functions,
and we don't show to the outside how a filter is actually implemented
(or what struct is behind it).

If we don't allow the higher-tier on the left side, we cannot chain
multiple or/and on the same level. Since or is associative, we shouldn't
expect the user to write (... or (... or ...)) and instead provide the
flattened version as well.

Otherwise they'd get tokenized as word and we couldn't build
conjunctions/disjunctions.