Add a couple of `/Since: 4.7.0.0/` annotations

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Export abstract `Text.Read.Lex.Number` type

This fixes some "could not find link destinations" Haddock warnings and seems to
the "right thing to do". The since-annotation for `Number` is changed as the `Number`
has becomes referencable in user code only w/ 4.7.0.0 even though it existed already
in 4.6.0.0.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>