Make operator() const-correct and add missing static functions. (PaddlePaddle#936)

* Make operator() const-correct and add missing static functions.

Currently, `*Converter::operator()` requires a mutable object to invoke,
and there are missing `static result_type convert(source_type const &
source)` overloads for certain partial specializations of `*Converter`
objects. This commit makes `operator()` const-correct and adds missing
function overloads where appropriate.

* minor changes

* format

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Co-authored-by: Haicheng Wu <[email protected]>

Author: Gregory-Meyer