Patches to make xi work better on Fuchsia

The debug plugins are disabled (with #cfg) on Fuchsia. Also, add
BUILD.gn build rules, so the core can be built using GN rather than
pulled in as a third-party dependency.

rust/core-lib/BUILD.gn

@@ -0,0 +1,9 @@
+import("//build/rust/rust_library.gni")
+
+rust_library("xi_core_lib") {
+    deps = [
+        "//rust/xi-editor/rust/rpc:xi_rpc",
+        "//rust/xi-editor/rust/rope:xi_rope",
+        "//rust/xi-editor/rust/unicode:xi_unicode",
+    ]
+}

rust/core-lib/src/plugins/manifest.rs

@@ -23,6 +23,7 @@ use syntax::SyntaxDefinition;
 static PLUGIN_DIR: &'static str = "XI_PLUGIN_DIR";
 
 // example plugins. Eventually these should be loaded from disk.
+#[cfg(not(target_os = "fuchsia"))]
 pub fn debug_plugins() -> Vec<PluginDescription> {
     use self::PluginActivation::*;
     use self::PluginScope::*;
@@ -60,6 +61,11 @@ pub fn debug_plugins() -> Vec<PluginDescription> {
         .collect::<Vec<_>>()
 }
 
+#[cfg(target_os = "fuchsia")]
+pub fn debug_plugins() -> Vec<PluginDescription> {
+    Vec::new()
+}
+
 /// Describes attributes and capabilities of a plugin.
 ///
 /// Note: - these will eventually be loaded from manifest files.

rust/rope/BUILD.gn

@@ -0,0 +1,5 @@
+import("//build/rust/rust_library.gni")
+
+rust_library("xi_rope") {
+
+}

rust/rpc/BUILD.gn

@@ -0,0 +1,5 @@
+import("//build/rust/rust_library.gni")
+
+rust_library("xi_rpc") {
+
+}

rust/unicode/BUILD.gn

@@ -0,0 +1,5 @@
+import("//build/rust/rust_library.gni")
+
+rust_library("xi_unicode") {
+
+}