Add Default implementation

[MattX]

Resolves #101.

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• Format
• No Std (1.31.0)
• No Std (stable)
• Test (1.31.0)
• Test (1.36.0)
• Test (beta)
• Test (nightly)
• Test (stable)