Refactor library to remove dependency on Position

The position of devices was previously managed by the Pica
context instance and updated by pica commands such as
SetPosition/InitUciDevice.

This change introduces the RangingEstimator trait, and adds
an instance of ranging estimator to the parameters of the
Pica::new() function.

Instead of relying on device positions, ranging measurements
are now produced by the owned instance of RangingEstimator.

The http server _which receives HTTP requests to update
device positions_ now caches the device positions instead of the
Pica context, and implements a RangingEstimator to pass
to the Pica context.

Cargo.toml

@@ -14,7 +14,7 @@ authors = [
   "David De Jesus Duarte <licorne@google.com>",
   "Henri Chataing <henrichataing@google.com>",
 ]
-default-run = "pica-server"
+default-run = "pica"
 exclude = [
   "res/*",
   "scripts/*"
@@ -28,8 +28,13 @@ name = "pica"
 path = "src/lib.rs"
 
 [[bin]]
-name = "pica-server"
-path = "src/bin/server/mod.rs"
+name = "pica"
+path = "src/bin/main.rs"
+
+[[bin]]
+name = "pica-http"
+path = "src/bin/http-server/main.rs"
+features = ["web"]
 
 [features]
 default = ["web"]