chore: replace ethlint with solhint

Author: mariocao

.solhint.json

@@ -0,0 +1,26 @@
+{
+  "extends": "solhint:recommended",
+  "rules": {
+    "compiler-version": [
+      "error",
+      ">=0.6.0 <0.9.0"
+    ],
+    "constructor-syntax": "warn",
+    "func-param-name-mixedcase": "warn",
+    "func-visibility": [
+      "warn",
+      {
+        "ignoreConstructors": true
+      }
+    ],
+    "modifier-name-mixedcase": "warn",
+    "no-empty-blocks": "off",
+    "no-inline-assembly": "off",
+    "reason-string": [
+      "warn",
+      {
+        "maxLength": 80
+      }
+    ]
+  }
+}

.solhintignore

@@ -0,0 +1,2 @@
+node_modules
+flattened

.soliumignore

@@ -17,15 +17,13 @@
     "compile:flattened": "npx truffle compile --all --config flattened-config.js ",
     "coverage": "solidity-coverage",
     "flatten": "node scripts/flatten.js",
-    "fmt": "solium -d contracts && eslint ./test",
-    "fmt!": "solium -d contracts --fix && eslint ./test --fix",
+    "fmt!": "solhint --max-warnings 0 \"contracts/**/*.sol\" --fix && solhint \"test/**/*.sol\" --fix && eslint ./scripts/*.js --fix && eslint \"test/**/*.js\" --no-error-on-unmatched-pattern --fix",
+    "fmt": "solhint --max-warnings 0 \"contracts/**/*.sol\" && solhint \"test/**/*.sol\" && eslint ./scripts/*.js && eslint \"test/**/*.js\" --no-error-on-unmatched-pattern",
     "lint": "eslint ./test",
     "lint:fix": "eslint ./test --fix",
     "migrate": "npx truffle migrate",
     "migrate:flattened": "npm run flatten && npx truffle migrate --config flattened-config.js",
     "postinstall": "node scripts/postinstall.js",
-    "solium": "solium -d contracts",
-    "solium:fix": "solium -d contracts --fix",
     "toolkit": "npx witnet-toolkit",
     "try": "npm run compile:requests && npx witnet-toolkit try-data-request --from-solidity ./contracts/requests/",
     "verify:flattened": "npx truffle run verify"
@@ -50,11 +48,11 @@
     "eslint-plugin-node": "^8.0.1",
     "eslint-plugin-promise": "^4.1.1",
     "eslint-plugin-standard": "^4.0.1",
-    "ethlint": "^1.2.5",
     "ganache-cli": "^6.8.2",
     "node-eval": "^2.0.0",
     "npx": "^10.2.2",
     "protocol-buffers": "^4.1.0",
+    "solhint": "3.3.4",
     "solidity-coverage": "^0.5.11",
     "truffle": "^5.1.10",
     "truffle-assertions": "^0.9.2",