chore: fix some warnings

Author: peter-jerry-ye

next/sources/error_codes/0013_fixed/example_1.mbt

@@ -3,5 +3,5 @@ fn example_explicit_variable_types() -> Unit {
   let a : Array[Int] = []
   println(a.length())
   let b : Int? = None
-  println(b.is_empty())
+  println(b is None)
 }

next/sources/error_codes/0013_fixed/example_2.mbt

@@ -3,5 +3,5 @@ fn example_annotated_collections() -> Unit {
   let a = ([] : Array[Int])
   println(a.length())
   let b = (None : Int?)
-  println(b.is_empty())
+  println(b is None)
 }

next/sources/error_codes/0017_fixed/example_0.mbt

@@ -1,5 +1,5 @@
 ///|
-test (ctx : @test.T) {
+test (ctx : @test.Test) {
   let println = ctx.writeln(_)
   // start example
   let a : StringView = "asdf"

next/sources/error_codes/0017_fixed/example_1.mbt

@@ -1,5 +1,5 @@
 ///|
-test (ctx : @test.T) {
+test (ctx : @test.Test) {
   let println = ctx.writeln(_)
   // start example
   let a : StringView = "asdf"

next/sources/error_codes/0018_error/top.mbt

@@ -1,5 +1,5 @@
 ///|
-test (ctx : @test.T) {
+test (ctx : @test.Test) {
   let println = ctx.writeln(_)
   // start example
   let count = 2

next/sources/error_codes/0018_fixed/example_0.mbt

@@ -1,5 +1,5 @@
 ///|
-test (ctx : @test.T) {
+test (ctx : @test.Test) {
   let println = ctx.writeln(_)
   // start example
   let count = 2

next/sources/error_codes/0018_fixed/example_1.mbt

@@ -1,5 +1,5 @@
 ///|
-test (ctx : @test.T) {
+test (ctx : @test.Test) {
   let println = ctx.writeln(_)
   // start example
   let count = 2

next/sources/language/src/trait/top.mbt

@@ -17,7 +17,7 @@ pub impl MyShow for MyType with to_string(self) {
   ...
 }
 
-struct MyContainer[T] {}
+struct MyContainer[_] {}
 
 // trait implementation with type parameters.
 // `[X : Show]` means the type parameter `X` must implement `Show`,