Allow synapse creation with arrays in standalone

As the title says, you are now allowed to create synaptic connections in standalone not only via string expressions but also with explicit connection arrays (using the static array mechanism). This allows to completely specify a network from stored data, also setting synaptic state variables from arrays is possible, if done without indexing (or equivalently with `[:]`).

Note that `Synapses.connect` allows to combine explicit connection arrays with the `p` and `n` parameters. I think this will be rarely used (you could always do it yourself on the arrays), so I did not bother implementing it in the generated code, it will be done in Python for all codegen targets including standalone (i.e. codegen only gets the final arrays, possibly randomly "thinned" or repeated from the initially given ones).

Closes #254