fix: handle subscripts correctly when nested in expr within array function call

[chrispcampbell]

The primary part of this fix was to keep track of the current array function in a separate variable, currentArrayFunctionName, and use that in a couple places instead of accessing the most direct currentFunctionName, which is not accurate in the case of nested expressions.

As part of this, I changed LOOKUP so that it returns :NA: when the given Lookup is NULL, which can happen in the case of sparse datasets. I made this change here since the new sumif test case uses a sparse matrix (i.e., it's purposely missing data in one dimension).

I also changed initMode to mode and made the values more explicit, because I found that the previous approach was not fine-grained enough and would cause lookups to sometimes be initialized redundantly (for example, if a lookup is referenced inside of initLevels). With this new approach, we're more clear about whether we are in "decl" mode, or a specific kind of "init" mode, or "eval" mode.

Fixes #46