Fix StaticArray#to_json

[Vendicated]

StaticArray.to_json currently results in a compiler error coming from the crystal stdlib:

In /usr/share/crystal/src/json/to_json.cr:10:15

 10 | to_json(json)
              ^---
Error: expected argument #1 to 'StaticArray(Int32, 1)#to_json' to be IO, not JSON::Builder

Overloads are:
 - Object#to_json(io : IO)
 - Object#to_json()

Minimal Reproduction Example:

require "json"

StaticArray[1].to_json

This seems to be caused by it using the generic Object.to_json. This PR fixes it by overwriting to_json for StaticArray to be the same as for normal arrays

[straight-shoota]

Thanks @Vendicated and congrats on your first contribution to Crystal!