Add service proc macro

[petekubiak]

The purpose of this PR is to simplify the process for defining services and characteristics in TrouBLE, as laid out in this issue: #130.
Services are defined as a struct decorated with the #[gatt_service(uuid = ...)] attribute and fields within the struct can be decorated with #[characteristic(<args>)] attributes to define them as characteristics.
In a similar way, the gatt server is defined as a struct with the #[gatt_server] attribute, where each of the fields is a service.
The server is then instantiated with the ::new() function which is generated by the attribute macro.

This PR lays the foundation for expanding the functionality of the server, services and characteristics such that all access to characteristics (e.g. set, get, notify, indicate) can be done through methods on the characteristics, accessed through the server.

[plaes]

@petekubiak Could you split out the uuid parser/generator macro as separate PR?

This would immensely improve implementing services with 128-bit Uuids:

    let mut uart_service = table
        // #[gatt_service(uuid = "6E400001-B5A3-F393-E0A9-E50E24DCCA9E")] 
        .add_service(Service::new(Uuid::Uuid128([
                    0x9e, 0xca, 0xdc, 0x24,
                    0x0e, 0xe5, 0xa9, 0xe0,
                    0x93, 0xf3, 0xa3, 0xb5,
                    0x01, 0x00, 0x40, 0x6e,
            ])));

    // #[characteristic(uuid = "6E400002-B5A3-F393-E0A9-E50E24DCCA9E", write)]
    let _ = uart_service.add_characteristic(Uuid::Uuid128([
                    0x9e, 0xca, 0xdc, 0x24,
                    0x0e, 0xe5, 0xa9, 0xe0,
                    0x93, 0xf3, 0xa3, 0xb5,
                    0x02, 0x00, 0x40, 0x6e,
            ]),
            &[
                CharacteristicProp::Write,
            ],
            &mut rx,
        );

[alexmoon]

You can write .add_service(Service::new(Uuid::Uuid128(0x6E400001_B5A3_F393_E0A9_E50E24DCCA9Eu128.to_le_bytes()))). No macro needed.

[petekubiak]

Following on from that my hope is that you wouldn't even need to call .add_service() or .add_characteristic() as the attribute macro takes care of that for you when you call <Server name>::new() (Server name being the name of the struct you have decorated with the #[gatt_server] attribute).

[lulf]

Looks great, thanks the work on this! I've left a few inline comments.

I noticed a commend saying the pub server field would be non-public with additional changes. Do you wish to merge this now or make additional changes to it?

[petekubiak]

Looks great, thanks the work on this! I've left a few inline comments.

I noticed a commend saying the pub server field would be non-public with additional changes. Do you wish to merge this now or make additional changes to it?

Thanks! I think we'd like to merge this one first and then do further development in a new PR (avoids this one getting too large :) )

[petekubiak]

Looks great, thanks the work on this! I've left a few inline comments.

I noticed a commend saying the pub server field would be non-public with additional changes. Do you wish to merge this now or make additional changes to it?

I've decided to set the visibilities back to what they were and instead implement getter functions for the fields I need. At least that way they are immutable.