Prepare for cortex-m-rt 0.7.5

[adamgreig]

Fixes #532.

[thejpster]

Before we do the releases, have we checked that the new macro works as expected and rejects mismatched hard fault signatures?

[diondokter]

Before we do the releases, have we checked that the new macro works as expected and rejects mismatched hard fault signatures?

Tests were added: https://github.com/rust-embedded/cortex-m/blob/master/cortex-m-rt/tests/compile-fail/hard-fault-bad-signature-3.rs
There are more too.

It doesn't pick up on unreleased macro crate releases because it defines the dependency with a path:

cortex-m/cortex-m-rt/Cargo.toml

Lines 21 to 22 in 4395bae

	[dependencies]
	cortex-m-rt-macros = { path = "macros", version = "=0.7.0" }

This is probably the correct way in this crate, but yeah it does make it easy to forget to release any new version of it.

[jonathanpallant]

I think one solution for this is for the main branch to be changed to have the version number N + 1 right after version N is released, and for any dependencies within the same repo to also use N + 1 as a version number.

[jamesmunns]

CC #535 - @jannic thinks it is important before we release

[jamesmunns]

Also noting that @newAM successfully yanked v0.7.4 - we should probably note this in the release docs that we are fixing those regression(s).