Get rid of special *in* variable (BREAKING!)

[borkdude]

The name *in* now collides with clojure.core/*in*, an unforeseen problem when babashka was initially just designed to deal with small one-line snippets. The value *in* is really a delay which, when realized, reads a line of text or EDN value from stdin. We will remove this magic behavior and expose user/stdin as a delay that users could have written themselves in normal Clojure.

This is a reaction to #171.

Option 0:
Only rename, no other changes, leave the magic.
The new syntax becomes:

$ echo 'hello' | bb 'stdin'
hello

or:

$ echo 'hello' | bb '<stdin>'
hello

(or some other name that we like best).

This option is the least breaking one.

Option 1:
The new syntax becomes:

$ echo 'hello' | bb '@stdin'
hello

We're not hiding the fact that we're using a delay in the implementation.

Option 2:
Deprecate a special variable for stdin. We have read-line anyway. But what do we do for reading EDN? People should use (edn/read *in*)?

$ echo 'hello\nthere' | bb '[(read-line) (read-line)]'
["hello" "there"]

For reading all EDN values from stdin:

$ echo '{:a 1} {:a 2}' | bb -e '(vec (take-while #(not= % ::eof) (repeatedly #(edn/read {:eof ::eof} clojure.core/*in*))))'
[{:a 1} {:a 2}]