refine list.env (fix #49).
no longer converts named vector to list.
only take list as symbols for evaluation.
revert list.env
support converting named vector to list
list.search cleans results with is.empty
remove default value of envir
formal is pairlist
use is.empty
refine lambda
reduce List header message and add List.header option
Merge branch 'fix-env' into 0.3