Provide `From` impls

[malobre]

I'd like to use this crate's serde_value::Value as a more generic version of serde_json::Value.
My use case is an HTTP API where the success response body is defined as such:

#[derive(serde::Serialize)]
struct SuccessBody {
    data: serde_json::Value,
}

impl SuccessBody {
    pub fn new(data: impl Into<serde_json::Value>) -> Self {
        let data = data.into();
        Self { data }
    }
}

Since most (all?) serialize_* are infallible, e.g:

serde-value/src/ser.rs

Lines 77 to 87 in 47190f5

	fn serialize_bool(self, v: bool) -> Result<Self::Ok, Self::Error> {
	Ok(Value::Bool(v))
	}
	fn serialize_i8(self, v: i8) -> Result<Self::Ok, Self::Error> {
	Ok(Value::I8(v))
	}
	fn serialize_i16(self, v: i16) -> Result<Self::Ok, Self::Error> {
	Ok(Value::I16(v))
	}

this should be trivial to implement.

I can submit a PR if you believe this is worth implementing.