Move and reformat hints (exercism#593)

Author: angelikatyborska

exercises/practice/diffie-hellman/.docs/hints.md

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+## General
+
+- For generating random numbers, Erlang's `:rand.uniform` or `Enum.random` are
+good places to start.
+- `:math.pow |> round` can be used to find the power of a number, and `rem` for
+the modulus.
+- Neither of those works particularly well (or quickly) for very large integers.
+Cryptography generally makes use of numbers larger than 1024 bits. Erlang's
+`:crypto` module has a useful function for finding the modulus of a power,
+particularly for enormous integers, but you might need `:binary` to decode it.

exercises/practice/diffie-hellman/hints/hints.md

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+## General
+
+- Use `Bitwise` (or div/rem).
+- If you use `Bitwise`, an easy way to see if a particular bit is set is to compare
+the binary AND (`&&&`) of a set of bits with the particular bit pattern you
+want to check, and determine if the result is the same as the pattern you're
+checking.

exercises/practice/secret-handshake/.docs/hints.md

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+## General
+
+- `apply` will let you pass arguments to a function, as will `fun.(args)`.

exercises/practice/secret-handshake/hints/hints.md

@@ -1,3 +1,5 @@
-Formatting the output is easy with `String`'s padding functions. All number
+## General
+
+- Formatting the output is easy with `String`'s padding functions. All number
 columns can be left-padded with spaces to a width of 2 characters, while the
 team name column can be right-padded with spaces to a width of 30.