Better subsequence support

[Zulu-Inuoe]

I've been asked several times for how to use parse on a part of an existing vector ala :start/:end can do with CL functions like find.

While jzon works with displaced arrays for this purpose, they have the downsides of

1. being incredibly unergonomic to construct: `(make-array (- (length array) start-offset) :element-type (array-element-type array) :displaced-to array :displaced-index-offset start-offset)
2. bringing a performance penalty on access

I'd like to address this somehow. The options I can think of atm are:

1. Add :start/:end to jzon:parse and jzon:make-parser
2. Add a jzon:span function to represent 'spans' of a sequence using an dedicated jzon data type
3. Add a jzon:span function to construct a displaced array for the user in a less painful way

Design-wise, I'm opposed to 1 on the basis that this argument only makes sense for a subset of inputs to parse and friends, and I'd like to further expand the inputs permitted in the future

From the perspective of the API, 2 and 3 are equivalent as the user sees the same: (jzon:parse (jzon:span seq :start 5 :end 10)).

Where the differences are more significant is that with 3, if we want to be fully compliant with the spec, there's not much room for optimization in the jzon:reader interface because technically the user could modify the input vector while reading.
That said, this is such an edge-case, that if a user encounters this issue it may be better to deal with it then and there.

I'm leaning on 2 to be conservative and future proof.