🎨 Introduce contextualized logging

This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.

Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.

Closes #265

vendor/github.com/ipfs/go-log/v2/levels.go

@@ -0,0 +1,30 @@
+package log
+
+import "go.uber.org/zap/zapcore"
+
+// LogLevel represents a log severity level. Use the package variables as an
+// enum.
+type LogLevel zapcore.Level
+
+var (
+	LevelDebug  = LogLevel(zapcore.DebugLevel)
+	LevelInfo   = LogLevel(zapcore.InfoLevel)
+	LevelWarn   = LogLevel(zapcore.WarnLevel)
+	LevelError  = LogLevel(zapcore.ErrorLevel)
+	LevelDPanic = LogLevel(zapcore.DPanicLevel)
+	LevelPanic  = LogLevel(zapcore.PanicLevel)
+	LevelFatal  = LogLevel(zapcore.FatalLevel)
+)
+
+// LevelFromString parses a string-based level and returns the corresponding
+// LogLevel.
+//
+// Supported strings are: DEBUG, INFO, WARN, ERROR, DPANIC, PANIC, FATAL, and
+// their lower-case forms.
+//
+// The returned LogLevel must be discarded if error is not nil.
+func LevelFromString(level string) (LogLevel, error) {
+	lvl := zapcore.InfoLevel // zero value
+	err := lvl.Set(level)
+	return LogLevel(lvl), err
+}