Implement `From<T> for Bson` for more integer types

[tyilo]

To make doc! { "foo": v } more ergonomic, it would be nice if the following types implemented infallible conversion to Bson:

Smaller integer types

i8, i16 -> Bson::Int32
u8, u16 -> Bson::Int32

NonZero<T> types where Bson: From<T>

This direction is lossless.

Platform-specific types

isize, usize -> Bson::Int64

I don't think that this should be added for compatibility reasons, but I wanted to add it as an option.