Change is_decodable to decodable?

[Strech]

To follow the Elixir naming convention

Type checks and other boolean checks that are allowed in guard clauses are named with an is_ prefix.

Examples: Integer.is_even/1, is_list/1

These functions and macros follow the Erlang convention of an is_ prefix, instead of a trailing question mark, precisely to indicate that they are allowed in guard clauses.

Note that type checks that are not valid in guard clauses do not follow this convention. For example: Keyword.keyword?/1.

[azeemchauhan]

@Strech I had free time, so addressed this enhancement in the PR #138