definite assignment analysis for let (#21024)

* draft for let daa

* patch

* fixes bugs

* errors for global let variable reassignments

* checkpoint

* out param accepts let

* add more tests

* add documentation

* merge tests

tests/init/tlet.nim

@@ -0,0 +1,34 @@
+{.experimental: "strictDefs".}
+
+proc bar(x: out string) =
+  x = "abc"
+
+proc foo() =
+  block:
+    let x: string
+    if true:
+      x = "abc"
+    else:
+      x = "def"
+    doAssert x == "abc"
+  block:
+    let y: string
+    bar(y)
+    doAssert y == "abc"
+  block:
+    let x: string
+    if true:
+      x = "abc"
+      discard "abc"
+    else:
+      x = "def"
+      discard "def"
+    doAssert x == "abc"
+  block: #
+    let x: int
+  block: #
+    let x: float
+    x = 1.234
+    doAssert x == 1.234
+static: foo()
+foo()

tests/init/tlet_uninit2.nim

@@ -0,0 +1,5 @@
+discard """
+  errormsg: "'let' symbol requires an initialization"
+"""
+
+let x: int

tests/init/tlet_uninit3.nim

@@ -0,0 +1,24 @@
+discard """
+  cmd: "nim check $file"
+  action: "reject"
+  nimout: '''
+tlet_uninit3.nim(13, 5) Error: 'let' symbol requires an initialization
+tlet_uninit3.nim(19, 5) Error: 'x' cannot be assigned to
+tlet_uninit3.nim(23, 11) Error: 'let' symbol requires an initialization
+'''
+"""
+
+{.experimental: "strictDefs".}
+
+let global {.used.}: int
+
+proc foo() =
+  block:
+    let x: int
+    x = 13
+    x = 14
+
+  block:
+    let x: int
+    doAssert x == 0
+foo()

tests/init/tlet_uninit4.nim

@@ -0,0 +1,12 @@
+discard """
+  errormsg: "type mismatch: got <string>"
+"""
+
+{.experimental: "strictDefs".}
+
+proc foo(x: var string) =
+  echo x
+
+proc bar() =
+  let x: string
+  foo(x)