add natural log to symbolics.jl

@exaexa is there a place we can view all the names and add more? we are probably missing a bunch

src/symbolics.jl

@@ -23,6 +23,7 @@ const default_symbolics_mapping = Dict{String,Any}(
     "ceiling" => :ceil,
     "floor" => :floor,
     "piecewise" => :(Core.ifelse),
+    "ln" => :log,
 )
 
 allowed_sym(x, allowed_funs) =