nanocoap: add coap_get_token()

[benpicco]

Contribution description

This adds a coap_get_token() function so we can eventually get rid of the token pointer in coap_pkt_t.

Testing procedure

Issues/PRs references

[kfessel]

 * @returns     pointer to the token-position

does not tell you about its lenght

[fjmolinas]

Anything missing @kfessel?

[maribu]

ACK. This is a nice one :)

[kfessel]

I am not sure about the name coap_get_token sounds a bit like you get a value so:
coap_token
coap_pkt_token
coap_pkt_token_ptr
coap_token_ptr

functionality is good (and marian just acked)

[benpicco]

We also have coap_get_token_len() so it fits right in.

[kfessel]

with coap_get_token_len() you get a value (not a ptr into the pktbuffer) (i seems kind o strange to have a get on the left side of a memcpy)

but maybe i am over thinking that naming thing,

lets move on an deprecate the use of coap_pkt_t.token