In RoutePathManager, the forward declaration of the RoutingContext was
incorrectly made a class instead of a struct. This was causing several
warnings in the Clang build (but not in the GCC build). Fixed since the
forward declaration should match the declaration.

This code previously used an int to calculate the size of an allocation,
which was done by shifting a bit; however, this is technically
ill-formed since this could generate a negative value. This was causing
a warning to appear in the most recent version of GCC. Although this is
technically ok, its best that this properly uses size_t instead.

The way this section of code was written could potentially lead to an
array access out of bounds. This was causing a warning in future
versions of GCC. Simply fixed this issue.

This line of code was likely a mistake; the src of a strcpy should never
be a freshly allocated pointer. Fixed the line to likely what the coder
intended.

GCC sometimes gets confused with how strings are allocated, when they
are C strings allocated by hand. Changed to an std string since it is
easier to work with and removes the warning.

When a class returns a reference to a dereferenced pointer, if the
pointer does not point to a const object, returning that pointer as a
const has no effect. This was causing warnings.

To fix this, we want a refernece to a const object not a const reference
to an object (silly, I know). So we need to make this more clear in the
class.

The -lz flag is used to link with the ZLIB library, but the library was
already being linked; so there was no reason to add the flag. This was
causing an insane number of warnings in the Clang build. If this is
added back for any reason, this should be checked.

Also cleaned up how ZLIB was being linked. ZLIB is required for reading
and processing the atom circuit; so it should always be linked with VPR.