Add support for parsing AbstractStrings to numbers

[Wynand]

Currently Base.parse takes any AbstractString string as a parameter, however internal methods are written only to support Union{String,Substring{String} strings.

This adds support for parsing any AbstractString (assuming it can be converted to String), but I believe it could be much more efficient

[simeonschaub]

Thanks for the PR!

I think it might be slightly more idiomatic to do this with dispatch instead, i.e. just define:

tryparse_internal(::Type{Bool}, sbuff::AbstractString) = tryparse_internal(Bool, String(sbuff)::String)

alongside the tryparse_internal(::Type{Bool}, sbuff::Union{String, SubString{String}}) one.

Would you mind also adding a test that reproduces the issue you encountered?