fix: improve sqrt implementations for f32 and f64 without std

[roderickvd]

The CI got broken when the no_std implementations for f32 and f64 were custom-rolled instead of using core intrinsics in this commit: cd8d893. Beyond breaking the CI, RMS values are way off.

Problem

The current implementation:

• takes an initial guess without refinement
• uses an incorrect bias for f64
• causing wildly inaccurate values:

failures:

---- dasp_rms/src/lib.rs - Rms<F,S>::next (line 113) stdout ----
Test executable failed (exit status: 101).

stderr:

thread 'main' (6233708) panicked at dasp_rms/src/lib.rs:10:5:
assertion `left == right` failed
  left: [5.593755989478844e-155]
 right: [0.5]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


---- dasp_rms/src/lib.rs - Rms<F,S>::current (line 172) stdout ----
Test executable failed (exit status: 101).

stderr:

thread 'main' (6233709) panicked at dasp_rms/src/lib.rs:10:5:
assertion `left == right` failed
  left: [2.631772124e-315]
 right: [0.0]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


---- dasp_rms/src/lib.rs - Rms<F,S>::reset (line 62) stdout ----
Test executable failed (exit status: 101).

stderr:

thread 'main' (6233711) panicked at dasp_rms/src/lib.rs:13:5:
assertion `left == right` failed
  left: [2.631772124e-315]
 right: [0.0]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


---- dasp_signal/src/rms.rs - rms::SignalRms::rms (line 26) stdout ----
Test executable failed (exit status: 101).

stderr:

thread 'main' (6236011) panicked at dasp_signal/src/rms.rs:13:5:
assertion `left == right` failed
  left: [[6.749798802814877e-155], [9.136468277877633e-155], [7.458341613357585e-155]]
 right: [[0.6363961030678927], [0.8514693182963201], [0.7071067811865476]]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Solution

This PR improves the initial guess and uses Newton-Raphson refinement to improve the accuracy to >6 decimal places.