Create C API for signal-estimator

[gavv]

A very needed feature is an ability to use latency estimation algorithm programmatically outside of signal-estimator CLI and GUI.

In addition to CLI (src/cli) and GUI (src/gui), we could add a C library (src/lib) that works on top of the same low-level C++ classes (src/base), same as CLI and GUI does.

(Note: C API is preferred over C++ API: it provides better ABI compatibility over library upgrades and across different compilers; it provides compatibility with more languages, since many languages allow to create bindings for C libraries quite easily)

The library does not need to work with ALSA devices. Instead, it should have a simpler interface:

• user creates estimator
• user obtains from it samples for playback
• user passes to it captured samples
• user reads estimated latency from estimator

Something like this:

typedef struct estimator_config {
    /* various configuration options: mode, sample rate, channel count, etc */
} estimator_config; 

typedef struct estimator_report {
    /* reported latency, loss ration, etc */
} estimator_report; 

/* opaque handle */
typedef struct estimator estimator;

estimator* estimator_open(const estimator_config* config);

void estimator_pop_output(estimator* estimator, float* samples, size_t num_samples);
void estimator_push_input(estimator* estimator, const float* samples, size_t num_samples);

void estimator_get_report(estimator* estimator, estimator_report* report);

void estimator_close(estimator* estimator);

To implement it, we'll need to use existing IEstimator implementation (depending on selected mode) with custom IReporter implementation that will store results in memory instead of printing them to console.

Important: this API should be thread-safe. It should be allowed to invoke all functions from different threads concurrently.