Moving `random` constructors, NDArray Constructor changes & New unit tests

[shivasankarka]

NDArray

• The constructors that take shape and fill value as input return a unintialized array by default if a fill value is not provided.
• The fill value operation is vectorized.
• Removed random argument from NDArray constructors.

RANDOM (Related to #96 ):

• Added rand(*shape: Int, min: Scalar[dtype], max: Scalar[dtype]) and rand(shape: List[Int], min: Scalar[dtype], max: Scalar[dtype]) for creating random arrays with random values within a range [min, max].

• Added randn(*shape: Int, mean: Scalar[dtype], variance: Scalar[dtype]) and rand(shape: List[Int], mean: Scalar[dtype], variance: Scalar[dtype]) for creating arrays with random values chosen from a normal distribution with given mean and variance. The name of this function could be changed to something else to remove any confusions.

Unit Tests

• Updated unit tests with new rand methods
• Added new unit tests files for NDArray constructors, array manipulation, boolean masks, slicing etc Notes:
• test_slicing.mojo doesn't work with .mojopkg in Mac currently.
• test_bool_mask.mojo has problems in comparison with numpy arrays.
• Not all tests are comprehensive, they are basic comparison checks against numpy arrays.

Cumulative reduce

• Changed the cumvariance function to return sigma (variance) instead of sigma^2.