Release 0.6.0 (uses NCS 1.5.1)

• Updated to nrfxlib-sys v1.5.1.
  • Requires Rust v1.51 as we use the new resolver to allow bindgen as a build-dep
  • Rename FFI exports from bsd_X to nrf_modem_X
  • Implement IPC functionality
  • Implement library and transmit heaps
  • Supply hard-float libraries by default. The nRF9160 has an FPU so we might as well use the VFP registers.
• Update to latest heapless - no more heapless::consts::Uxx