async: add i2c

[Dirbaio]

This mirrors the blocking i2c trait (including the changes proposed in #339 ), except the following:

• Removed all iter methods since they don't play well with DMA.

[rust-highfive]

r? @ryankurte

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[bors[bot]]

:-1: Rejected by too few approved reviews

[eldruin]

What? I just approved this

[bors[bot]]

Build succeeded:

• ci-linux (1.46.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)
• ci-linux-test (1.46.0, x86_64-unknown-linux-gnu)
• ci-linux-test (stable)
• fmt