rename lenTuple and lenVarargs (#13639)

* rename 'lenTuple' to 'tupleLen'

Rationale:
`lenTuple` is a tuple consisting of lengths (e.g. `(1, 5, 0)`),
`tupleLen` is a length of a tuple (e.g. `tupleLen((1, 5, 0) == 3`)

* rename 'lenVarargs' to 'varargsLen'

The same rationale as a previous commit. Consistency.

tests/system/tlenvarargs.nim

@@ -1,59 +0,0 @@
-discard """
-  output: '''
-tlenvarargs.nim:35:9 (1, 2)
-tlenvarargs.nim:36:9 12
-tlenvarargs.nim:37:9 1
-tlenvarargs.nim:38:8 
-done
-'''
-"""
-## line 10
-
-template myecho*(a: varargs[untyped]) =
-  ## shows a useful debugging echo-like proc that is dependency-free (no dependency
-  ## on macros.nim) so can be used in more contexts
-  const info = instantiationInfo(-1, false)
-  const loc = info.filename & ":" & $info.line & ":" & $info.column & " "
-  when lenVarargs(a) > 0:
-    echo(loc, a)
-  else:
-    echo(loc)
-
-template fun*(a: varargs[untyped]): untyped =
-  lenVarargs(a)
-
-template fun2*(a: varargs[typed]): untyped =
-  a.lenVarargs
-
-template fun3*(a: varargs[int]): untyped =
-  a.lenVarargs
-
-template fun4*(a: varargs[untyped]): untyped =
-  len(a)
-
-proc main()=
-  myecho (1, 2)
-  myecho 1, 2
-  myecho 1
-  myecho()
-
-  doAssert fun() == 0
-  doAssert fun('a') == 1
-  doAssert fun("asdf", 1) == 2
-
-  doAssert fun2() == 0
-  doAssert fun2('a') == 1
-  doAssert fun2("asdf", 1) == 2
-
-  doAssert fun3() == 0
-  doAssert fun3(10) == 1
-  doAssert fun3(10, 11) == 2
-
-  ## shows why `lenVarargs` can't be named `len`
-  doAssert fun4("abcdef") == len("abcdef")
-
-  ## workaround for BUG:D20191218T171447 whereby if testament expected output ends
-  ## in space, testament strips it from expected output but not actual output,
-  ## which leads to a mismatch when running test via megatest
-  echo "done"
-main()