Support #bodx in number parser

Support #bodx in number parser

`where radix = ...` returns a function that takes a number and parses it
accordingly. The function is chosed by 'try'ing to read a '#' followed
by any of "bodx" and if that fails assume the base is decimal.

The `>>` sequencing operator is used to combine `char` and `oneOf`, they
are executed one after another, discarding the value of the first.

`readInt` takes a base (2), a predicate returning what chars are valid
and a function that describes the Integer value for a char.
`(\x -> elem x "01")` is a lambda that returns true if x is in "01".

`(read . (:[]))` is interesting. The `.` operator composes the functions
`read` and `:[]`, where `:` is the append to list function and `[]` is
the "first argument", i.e. `:[]` appends an element to the empty list.

`let ((a,_):_) = ...` extracts the number from the `[(Number,String)]`
type returned by the `read*` functions.

Because strings starting with '#' are valid Atoms as well, we need to
parse for numbers first.