Base.show(::IO, ::T) does not return parseable Julia code for numerous types

[brainandforce]

From the docstring of Base.show:

The representation used by show generally includes Julia-specific formatting and type information, and should be parseable Julia code when possible.

Calling show on units or unitful values provided by this package does not provide parseable Julia code for the value:

julia> show(stdout, u"km/s")
km s^-1

julia> show(stdout, 4u"km/s")
4 km s^-1

I'd expect something along the lines of

julia> show(stdout, u"km/s")
u"km * s^-1"

julia> show(stdout, 4u"km/s")
4u"km * s^-1"

Fixing this should not affect the pretty printing methods, which use show(::IO, ::MIME"text/plain", ::T).

[giordano]

Duplicate of #412