Convert MultiTypeSymbolTable from storing nilable objects to non-nilable

[bradcray]

This issue notes that the current symbol table stores SymEntry? entries when it seems as though it could/should store (non-nilable) SymEntry entries instead. We should convert to the non-nilable version to reduce the need for nil checks altogether. This can be done at present, but only by abusing the language as noted in chapel-lang/chapel#14367.

This issue also relates to #169 in that using a map seems like a nice replacement for the associative domain/array combo that's currently used, but likely runs into chapel-lang/chapel#14361.

For this reason, I'm marking this as being in the Chapel team's court for now.

[glitch]

Issue cleanup: I'm going to close this one since the current code uses a Map with GenSymEntry (non-nillable) and we are expected to merge #786 which changes the map signature to AbstractSymEntry with a loose typing hierarchy.