Update blog to use build-std feature instead of cargo-xbuild

[phil-opp]

This updates the blog for the changes in #835. The changes on the blog include:

• Introduce the build-std feature of cargo instead of using cargo-xbuild crate 🎉
• Replace all instances of cargo xbuild/xrun/xtest with the standard cargo build/run/test across the second edition posts
• Add a dependency on rlibc to provide memcpy etc implementations.
• Update bootloader dependency to version 0.9.8
• Update the 'Heap Allocation' post to enable the alloc dependency in the unstable.build-std list in cargo/config.toml.
• Rename .cargo/config file to .cargo/config.toml