Leverage kotlinx-datetime for date and time API

For now, common classes are copied from
https://github.com/Kotlin/kotlinx-datetime and
WASI implementation of currentTime() is provided.

Timezones are not managed yet.

Closes #9

samples/wasi-sample/src/wasmMain/kotlin/WasiSample.kt

@@ -14,12 +14,13 @@
  * limitations under the License.
  */
 
+import kotlinx.datetime.Clock
 import org.kowasm.wasi.*
 
 fun main() {
     val wasi: Wasi = DefaultWasi
     wasi.println("Hello, world!")
-    wasi.println(wasi.now().toString())
+    wasi.println(Clock.System.now().toString())
     wasi.createDirectory("testDir")
     wasi.createFile("testFile")
     wasi.listDirectoryEntries(".").forEach(wasi::println)

wasi/src/commonMain/kotlin/kotlinx/datetime/Clock.kt

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2019-2020 JetBrains s.r.o.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime
+
+import kotlin.time.*
+
+/**
+ * A source of [Instant] values.
+ *
+ * See [Clock.System][Clock.System] for the clock instance that queries the operating system.
+ */
+public interface Clock {
+    /**
+     * Returns the [Instant] corresponding to the current time, according to this clock.
+     */
+    public fun now(): Instant
+
+    /**
+     * The [Clock] instance that queries the operating system as its source of knowledge of time.
+     */
+    public object System : Clock {
+        override fun now(): Instant = @Suppress("DEPRECATION_ERROR") Instant.now()
+    }
+
+    public companion object {
+
+    }
+}
+
+/**
+ * Returns the current date at the given [time zone][timeZone], according to [this Clock][this].
+ */
+public fun Clock.todayIn(timeZone: TimeZone): LocalDate =
+    now().toLocalDateTime(timeZone).date
+
+/**
+ * Returns a [TimeSource] that uses this [Clock] to mark a time instant and to find the amount of time elapsed since that mark.
+ */
+@ExperimentalTime
+public fun Clock.asTimeSource(): TimeSource = object : TimeSource {
+    override fun markNow(): TimeMark = InstantTimeMark(now(), this@asTimeSource)
+}
+
+@ExperimentalTime
+private class InstantTimeMark(private val instant: Instant, private val clock: Clock) : TimeMark {
+    override fun elapsedNow(): Duration = clock.now() - instant
+
+    override fun plus(duration: Duration): TimeMark = InstantTimeMark(instant + duration, clock)
+
+    override fun minus(duration: Duration): TimeMark = InstantTimeMark(instant - duration, clock)
+}
+
+@Deprecated("Use Clock.todayIn instead", ReplaceWith("this.todayIn(timeZone)"), DeprecationLevel.WARNING)
+public fun Clock.todayAt(timeZone: TimeZone): LocalDate = todayIn(timeZone)