Allow spell circles to access player inventory

[vgskye]

fixes #638

[SamsTheNerd]

this code looks basically copy pasted from the player casting env, maybe it should just be abstracted into a shared method somewhere?

[vgskye]

it is based off of that, but there are some behavioural changes(e.g. pull mainhand first then offhand, rather than other-hand-first or other-hand-only since spell circles are hands-free) that I'm not sure makes sense to unify into one set of methods