add invocant to glossary

[gfldex]

This invented word invokes confusion: https://irclog.perlgeek.de/perl6/2017-09-16#i_15171710

[jonathanstowe]

All words are made up but as computer terms go it's actually used quite close to the dictionary definition https://www.merriam-webster.com/dictionary/invocant - hardly any metaphorical leap required :)

[AlexDaniel]

@jonathanstowe yes, but even if so, it would be nice to explain it for everyone who may not know the definition or expect that the definition can be looked up in a regular dictionary (that includes non-native speakers and possibly six-year-olds). It's not the most common word after all.