NFC refactoring (#3050)

"A long time ago in a galaxy far, far away...." we started NFC subsystem refactoring.

Starring:

- @gornekich - NFC refactoring project lead, architect, senior developer
- @gsurkov - architect, senior developer
- @RebornedBrain - senior developer

Supporting roles:

- @skotopes, @DrZlo13, @hedger - general architecture advisors, code review
- @Astrrra, @doomwastaken, @Hellitron, @ImagineVagon333 - quality assurance

Special thanks:

@bettse, @pcunning, @nxv, @noproto, @AloneLiberty and everyone else who has been helping us all this time and contributing valuable knowledges, ideas and source code.

lib/digital_signal/digital_signal_i.h

@@ -0,0 +1,23 @@
+/**
+ * @file digital_signal_i.h
+ * @brief DigitalSignal private definitions.
+ *
+ * This file is an implementation detail. It must not be included in
+ * any public API-related headers.
+ */
+#include <stdint.h>
+#include <stdbool.h>
+
+#define DIGITAL_SIGNAL_T_TIM 1562 /**< 15.625 ns *100 */
+#define DIGITAL_SIGNAL_T_TIM_DIV2 (DIGITAL_SIGNAL_T_TIM / 2) /**< 15.625 ns / 2 *100 */
+
+/**
+ * @brief DigitalSignal structure type.
+ */
+struct DigitalSignal {
+    bool start_level; /**< The level to begin the signal with. */
+    uint32_t size; /**< Current period count contained in the instance. */
+    uint32_t max_size; /**< Maximum period count this instance can hold. */
+    uint32_t* data; /**< Pointer to the array of time periods. */
+    int32_t remainder; /**< Remainder left after converting all periods into timer ticks. */
+};