Mark unstable APIs

[MabezDev]

We need to mark APIs we're not stabilizing as such. These are all APIs except following:

• esp_hal::init
• esp_hal::Config (and any configs included within that)
• clocks
• time - Dependant on 1.0 release korken89/fugit#52
• interrupt
• peripheral
• peripherals
• prelude
• gpio - Stable driver analysis: GPIO #2492
• uart - Stable driver analysis: UART #2495
• spi::master::Spi - Stable driver analysis: SPI master #2494
• i2c::master::I2c - Stable driver analysis: I2C master #2493

We should just have one feature unstable. I thought about a feature per driver/module but in reality enabling all or just a few won't make a difference to the end user, if we make a breaking change to an unstable API it will only affect users who are actively using that API, not just because its available.

How we implement unstable is being tracked here: #2498