Add array methods to remove a range of elements and a position and count

Signatures of added methods:
proc remove(position: this._value.idxType, count: this._value.idxType) where chpl__isDense1DArray();
proc remove(positions: range(this._value.idxType, stridable=false)) where chpl__isDense1DArray();

Changed the chpl__isVectorSafe() helper method to chpl__isDense1DArray().

Added a new case to the arrayRemove test.