Prepare for 2.1.5 release

new releasse

Bump version to 2.1.4

Bump version to 2.1.3

Bump version to 2.1.2

Prepare 2.1.1 release

Bump version to 2.1.0

Bump version to 2.0.2

Bump version to 2.0.1

Make round-tripping Decode.Value possible

This change introduces the Encode.value function to encode a
Decode.Value into a string. This also necessitated a type name change
from Encode.Value (which was a different type from Decode.Value) to
Encode.Encoder. This better captures the purpose of Encode.Encoder
and does not lead to confusing name clashes.

This is a major change so version is bumped to 2.0.0.

This also closes #21