rm TaintedString for string; some proc -> func

[tersec]

• use plain string everywhere in place of deprecated https://nim-lang.org/docs/system.html#TaintedString which generates lots of warnings when building Nimbus with Nim 1.6.

In Nim 1.6, a TaintedString is a string:

type TaintedString* {.deprecated: "Deprecated since 1.5".} = string

so this has no effect.

In Nim 1.2 and Nim 1.4, it's a normal string unless taintMode is defined, which at least in nimbus-eth2, does not happen:

when taintMode:
  type TaintedString* = distinct string ## A distinct string type that
                                        ## is `tainted`:idx:, see `taint mode
                                        ## <manual_experimental.html#taint-mode>`_
                                        ## for details. It is an alias for
                                        ## ``string`` if the taint mode is not
                                        ## turned on.

Furthermore, the all the TaintedString/string conversions I found did no validity checking at all, just treated this as one more (in reality, nonexistent) roadbump to bulldoze through the type system. This includes all the couple dozen Status Nimbus git submodules that nimbus-eth2 uses.

• use func for top/module-level functions where possible instead of proc.

• remove some use of result = returns in simple cases in parseCmdArg overloads.