Fix $TraceBuiltin behavior

[mmatera]

This fixes #1481. With this changes:

In[1]:= $TraceBuiltins=False
Out[1]= False

In[2]:= $TraceBuiltins
Out[2]= False

In[3]:= $TraceBuiltins=True
Out[3]= True

In[4]:= $TraceBuiltins
Out[4]= True

In[5]:= $TraceBuiltins=True;$TraceBuiltins=True;$TraceBuiltins=False;ClearTrace[];PrintTrace[]
count     ms Builtin name

In[6]:= 

[rocky]

LGTM.

With this, are we ready for release?

@mmatera There are some small documentation changes, and I would like to go over the entire mathics.pdf, but other than that and release notes, these are all the changes I envision. And you?

[mmatera]

I think so. There is just another thing I would like to include on this, and is to replace this print() to show the PrintTrace[] results, by a proper (WL) Print.