Properly emit and consume special numerical/boolean R values

- special R values: Inf, -Inf, NaN, NA, TRUE, and FALSE
- because the YAML specification does not specify how to handle NA values,
  the various NA types are emitted as follows:
  - NA: .na
  - NA_real_: .na.real
  - NA_integer_: .na.integer
  - NA_character_: .na.character