Address review comments; Add documentation; Shared buffering mechanism for input and output streams

tests/test_inputs.nim

@@ -0,0 +1,36 @@
+{.used.}
+
+import
+  os, unittest, strutils, stew/ranges/ptr_arith,
+  ../faststreams
+
+suite "input stream":
+  test "empty input":
+    var str = ""
+    var i = unsafeMemoryInput(str)
+
+    check:
+      i.readable == false
+      i.next.isNone
+
+    expect Defect:
+      echo i.read
+
+  test "missing file input":
+    const fileName = "there-is-no-such-faststreams-file-1"
+
+    check not fileExists(fileName)
+    expect CatchableError: discard fileInput(fileName)
+
+    check not fileExists(fileName)
+    expect CatchableError: discard memFileInput(fileName)
+
+    check not fileExists(fileName)
+
+  test "simple":
+    var input = repeat("1234 5678 90AB CDEF\n", 1000)
+    var stream = unsafeMemoryInput(input)
+
+    check:
+      (stream.read(4) == "1234".toOpenArrayByte(0, 3))
+