Compilation (type mismatch) error with v0.2.6
Trying to compile with Nim 0.19.4 leads to a type error:

```
mosdepth.nim(366, 18) Error: type mismatch: got <coverage_t, uint32>
but expected one of:
proc `[]`[A, B](t: OrderedTableRef[A, B]; key: A): var B
proc `[]`[T](p: SafeCPtr[T]; k: int): T
proc `[]`[A, B](t: TableRef[A, B]; key: A): var B
proc `[]`(fai: Fai; i: int): string
proc `[]`[A](t: var CountTable[A]; key: A): var int
proc `[]`[Idx, T](a: array[Idx, T]; i: BackwardsIndex): T
proc `[]`[A, B](t: var OrderedTable[A, B]; key: A): var B
proc `[]`[T](s: var openArray[T]; i: BackwardsIndex): var T
proc `[]`[A, B](t: var Table[A, B]; key: A): var B
proc `[]`(g: Genotypes; i: int): Genotype
proc `[]`[T, U, V](s: openArray[T]; x: HSlice[U, V]): seq[T]
proc `[]`[Idx, T](a: var array[Idx, T]; i: BackwardsIndex): var T
proc `[]`[A, B](t: Table[A, B]; key: A): B
proc `[]`[I: Ordinal; T](a: T; i: I): T
proc `[]`[A](t: CountTableRef[A]; key: A): var int
template `[]`(s: string; i: int): char
proc `[]`[A, B](t: OrderedTable[A, B]; key: A): B
proc `[]`[Idx, T, U, V](a: array[Idx, T]; x: HSlice[U, V]): seq[T]
proc `[]`(c: Cigar; i: int): CigarElement
proc `[]`[A](t: CountTable[A]; key: A): int
proc `[]`[T](s: openArray[T]; i: BackwardsIndex): T
proc `[]`(s: string; i: BackwardsIndex): char
proc `[]`[T, U](s: string; x: HSlice[T, U]): string
proc `[]`(v: Value; i: int): string

expression: [](vals, i)
```