PGI has problems with elpa_int_value_to_string() where we return a
variable-length string pointer. A workaround for now is to copy the string into
a non-pointer variable of the same length.

This closes issue #55

This prevented compilation with Intel 2018 Beta and solves issue #54. It
turns out you can only override type-bound procedures that are
accessible in the module where you override them, thus they have to be
private. A "deferred, private" type bound procedure could only be
overriden from the same module (or a submodule thereof).

Many thanks to Igor Vorobtsov from Intel for clarifying this.

Was introduced recently, when I moved the private headers to src/

That way, this can easily be re-used for interactive testing as well

Pavel noted that some header files were missing from the distribution.
Also, I moved the private generated header files to src/ in order to
more clearly separate them.