make splice! work when replacement is a length-1 generator

[adienes]

the contract for splice! asks for an "ordered collection." Looking at the implementation, empirically that seems to require length and iteration, and in the case of a length-1 replacement, also getindex. This PR removes that last restriction and concretely, allows replacement to be a length-1 generator.

before PR:

julia> arr = Any[1,2,3,4]
4-element Vector{Any}:
 1
 2
 3
 4

julia> splice!(arr, 2, (i for i in 'a'))
ERROR: MethodError: no method matching getindex(::Base.Generator{Char, typeof(identity)}, ::Int64)

after PR:

julia> arr = Any[1,2,3,4]
4-element Vector{Any}:
 1
 2
 3
 4

julia> splice!(arr, 2, (i for i in 'a'))
2