Serialize log values to json

[simao]

Use the kv_unstable_serde feature from log to format values as json, instead of quoting all values.

Previously, numeric or boolean values were formatted as strings, enclosed with double quotes. This is also mentioned in #30

For example (note the time field):

{"level":30,"time":"1649168203900","msg":"--> Response sent","method":"POST","path":"/sensors/inside_weather","status":"200 - OK","duration":"13.777815ms"}

With this change, fields are formatted without quotes, if possible:

{"level":30,"time":1649168203900,"msg":"--> Response sent","method":"POST","path":"/sensors/inside_weather","status":"200 - OK","duration":"13.777815ms"}

This makes filtering with tools like kibana and jq easier. (And jaxe ;))

This also properly formats values in case the app calling log is using nested values, i.e. when value is another json object or a json array. Previously femme would just double quote the json object or array.

Closes #30

[lrlna]

ah! thank you, this is great