Make Legolas._schema_version_from_record_type part of the API

[omus]

I found myself using this internal function when attempting to identify the schema version from the element type of an iterator:

record_type = Base.@default_eltype(records)
record_type <: AbstractRecord || error("Unable to determine record type from iterator")
schema_version = Legolas._schema_version_from_record_type(record_type)

[ararslan]

+1 from me, but note that this was intentionally omitted from the API; see #78.