Moving embedded-hal 0.2 traits to separate package

[ryan-summers]

This PR migrates the existing embedded-hal traits into the hal_02 specifier so that we can continue with freely adding in new e-h 1.0 implementations as they are desired.

[ryan-summers]

@richardeoin This isn't actually adding in any new features, but cleans up the repo in preparation for adding in 1.0 variants as well :)

[adam-hott]

This code builds successfully with the following warnings:

   Compiling stm32h7xx-hal v0.16.0 (/Users/hott/eng/stm32h7xx-hal)
warning: unexpected `cfg` condition value: `stm32h757cm7`
   --> src/lib.rs:114:7
    |
114 | #[cfg(feature = "stm32h757cm7")]
    |       ^^^^^^^^^^--------------
    |                 |
    |                 help: there is a expected value with a similar name: `"stm32h747cm7"`
    |
    = note: expected values for `feature` are: `can`, `chrono`, `cm4`, `cm7`, `crc`, `default`, `defmt`, `device-selected`, `dsi`, `embedded-display-controller`, `embedded-sdmmc`, `ethernet`, `example-ldo`, `example-smps`, `fdcan`, `fmc`, `gpio-h72`, `gpio-h747`, `gpio-h7a2`, `log`, `log-itm`, `log-rtt`, `log-semihost`, `ltdc`, `rand`, `rand_core`, `revision_v`, `rm0399`, `rm0433`, `rm0455`, `rm0468`, `rt`, `rtc`, `sdio-host`, and `sdmmc` and 20 more
    = help: consider adding `stm32h757cm7` as a feature in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

warning: trait `PulseDensityPinD2` is never used
  --> src/sai/pdm.rs:44:11
   |
44 | pub trait PulseDensityPinD2<SAI> {}
   |           ^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: trait `PulseDensityPinD3` is never used
  --> src/sai/pdm.rs:45:11
   |
45 | pub trait PulseDensityPinD3<SAI> {}
   |           ^^^^^^^^^^^^^^^^^

warning: trait `PulseDensityPinD4` is never used
  --> src/sai/pdm.rs:46:11
   |
46 | pub trait PulseDensityPinD4<SAI> {}
   |           ^^^^^^^^^^^^^^^^^

warning: trait `PulseDensityPinCK2` is never used
  --> src/sai/pdm.rs:48:11
   |
48 | pub trait PulseDensityPinCK2<SAI> {}
   |           ^^^^^^^^^^^^^^^^^^

warning: trait `PulseDensityPinCK3` is never used
  --> src/sai/pdm.rs:49:11
   |
49 | pub trait PulseDensityPinCK3<SAI> {}
   |           ^^^^^^^^^^^^^^^^^^

warning: trait `PulseDensityPinCK4` is never used
  --> src/sai/pdm.rs:50:11
   |
50 | pub trait PulseDensityPinCK4<SAI> {}
   |           ^^^^^^^^^^^^^^^^^^

warning: struct `Instant` is never constructed
  --> src/time.rs:82:12
   |
82 | pub struct Instant {
   |            ^^^^^^^

warning: `stm32h7xx-hal` (lib) generated 8 warnings