Add error structs to give more information on errors

[gordon-klotho]

Adds the following error structs:

• VertexAlreadyExistsError[K, T]
• VertexNotFoundError[K]
• EdgeAlreadyExistsError[K]
• EdgeNotFoundError[K]
• VertexHasEdges[K]
• EdgeCausesCycleError[K]

This may be breaking if users check err == instead of errors.Is for specific errors (such as err == ErrVertexNotFound and not errors.Is(err, ErrVertexNotFound). Depending on versioning strategy and definition of a breaking change, this may warrant a new major version.

This change also fixes a few issues I found in memoryStore:

• AddEdge did not conform to the Store interface: "If either vertex doesn't exit, ErrVertexNotFound should be returned for the respective vertex"
• UpdateEdge had a data race in which the edge could have changed between the s.Edge call and the setting of the edges due to not holding onto the lock.
• CreatesCycle was not read locking before accessing s.inEdges

To let the store handle error cases when it is part of its interface definition, undirected and directed:

• AddEdge don't check the vertices for existence
• RemoveEdge don't check edge existence

And a few minor changes I made while I was there:

• Use ListEdges instead of AdjacencyMap for Size, saves some allocations and another pass through the edges to create the map
• undirected copy directed's createsCycle method to delegate to the store's version for a more performant implementation