Collect serializations of complex vectors
Implement complex vector (CPLXSXP) class in the REXP hierarchy
Add test cases for complex vectors
Implement parsing of complex vectors (closes #30)
Support both types of special row names (closes #39)

There are two forms of compact encodings for row names when they're
consecutive integers 1..n: when automatic (e.g., data frame defaults),
it's `c(NA, -n)`, and when explicitly set it's `c(NA, n)`.