Provide signature for local functions

[renkun-ken]

Currently, we provide signature for functions defined in packages and global namespace. We could use XML parse data to also provide signature for locally defined functions (aka closures).

foo <- function(var1) {
  bar <- function(var2) {
    var1 + var2
  }
  bar(|)
}

where cursor | is at bar(|), the signature of bar can be extracted from XML parse data and provided to user.