Fix deprecation warning while deserializing BigFloat (and possibly other Big types)

[amitmurthy]

julia> io=IOBuffer(); serialize(io, BigFloat(1.0)); seekstart(io); deserialize(io);
WARNING: BigFloat(s::AbstractString) is deprecated, use parse(BigFloat,s) instead.
 in depwarn at ./deprecated.jl:63
 in call at deprecated.jl:49
 in deserialize at serialize.jl:702
 in deserialize_datatype at serialize.jl:636
 in handle_deserialize at serialize.jl:457
 in deserialize at serialize.jl:429
 in deserialize at serialize.jl:426
while loading no file, in expression starting on line 0

Need to review other "Big" types that may have the same issue and also add ser/deser tests for these types.