Use Cargo workspace

Cargo workspace in the root will help make the IDE experience universal.
Cargo.lock and the target directory for build artifacts will be in the root
of the mempool repository (with ./target ignored by git).

.gitignore

@@ -5,3 +5,4 @@ backend/mempool-config.json
 *.swp
 frontend/src/resources/config.template.js
 frontend/src/resources/config.js
+target

.vscode/settings.json

@@ -1,8 +1,5 @@
 {
   "editor.tabSize": 2,
   "typescript.preferences.importModuleSpecifier": "relative",
-  "typescript.tsdk": "./backend/node_modules/typescript/lib",
-  "rust-analyzer.linkedProjects": [
-    "./backend/rust-gbt/Cargo.toml"
-  ]
+  "typescript.tsdk": "./backend/node_modules/typescript/lib"
 }

Cargo.toml

@@ -0,0 +1,4 @@
+[workspace]
+members = [
+	"./backend/rust-gbt",
+]

backend/rust-gbt/.gitignore

@@ -1,4 +1,3 @@
-target
 index.node
 **/node_modules
 **/.DS_Store