Rewrite signal module in Rust (#124)

The second chunk of changes from
#122.

This re-implements the GNSS signal related types of functionality in
native Rust

## What's been changed

- The `signal` module has been broken up into multiple files, though
these extra files are not visible via the public API
- The various string conversion functions have been moved to utilize the
derive macros from the [`strum`
crate](https://docs.rs/strum/0.27.2/strum/index.html), but the actual
string values have remained the same
- Conversions to/from `swiftnav-sys` integer types has been replaced
with conversions to/from native Rust integer types
- The conversion error types have changed to no longer contain the
`swiftnav-sys` types and instead contain native Rust types
- The error types use the [`thiserror`
crate](https://docs.rs/thiserror/latest/thiserror/) to reduce boiler
plate code

## What's been removed

- `Code::sig_count()`
- `Code::chip_count()`
- `Code::chip_rate()`

## What's been added

- The `signal::consts` module
- `Constellation::sat_count()`
- `Constellation::first_prn()`
- `Code::get_carrier_frequency()`
- `Code::get_glo_channel_frequency()`
- `GnssSignal::carrier_frequency()`
- `GnssSignal::get_glo_channel_frequency()`

---------

Co-authored-by: Alejandro Duarte <[email protected]>

Author: jbangelo

swiftnav/Cargo.toml

@@ -14,6 +14,7 @@ rustversion = "1.0"
 chrono = { version = "0.4", optional = true }
 swiftnav-sys = { version = "^0.10.0", path = "../swiftnav-sys/" }
 strum = { version = "0.27", features = ["derive"] }
+thiserror = "2.0"
 
 [dev-dependencies]
 float_eq = "1.0.1"

swiftnav/src/lib.rs

@@ -63,6 +63,7 @@ pub mod edc;
 pub mod ephemeris;
 pub mod geoid;
 pub mod ionosphere;
+pub mod math;
 pub mod navmeas;
 pub mod reference_frame;
 pub mod signal;

swiftnav/src/math.rs

@@ -0,0 +1,18 @@
+// Copyright (c) 2025 Swift Navigation Inc.
+// Contact: Swift Navigation <[email protected]>
+//
+// This source is subject to the license found in the file 'LICENSE' which must
+// be be distributed together with this source. All other rights reserved.
+//
+// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND,
+// EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A PARTICULAR PURPOSE.
+
+/// We define a `const` max function since [`std::cmp::max`] isn't `const`
+pub(crate) const fn compile_time_max_u16(a: u16, b: u16) -> u16 {
+    if b < a {
+        a
+    } else {
+        b
+    }
+}