Implement string overloading for Object IDs

[bigs]

Reference: https://idris2.readthedocs.io/en/latest/reference/overloadedlit.html

If this is possible with FFI, we can simply reuse our existing gitOidFromString function. If that (understandably) is not possible, it would not be too difficult to implement object IDs in pure Idris 2 (with the help of MarcelineVQ's bytes library) with marshalling to and from the C type.