[stdlib] Add `DType.get_dtype()`

[martinvuyk]

Add DType.get_dtype().

This will enable a lot of vector based optimizations for more generic function signatures.

This is a split-off from PR #3577

Examples:

fn memset[type: Copyable](ptr: UnsafePointer[type], value: type, count: Int):
    alias dt = DType.get_dtype[type]()

    @parameter
    if dt is not DType.invalid:
        var p = ptr.bitcast[Scalar[dt]]()
        _memset_impl[dt](p, rebind[Scalar[dt]](value), count)
    else:
        for i in range(count):
            (ptr + i).init_pointee_copy(value)

[lattner]

The _from_str fixes seem "obviously good" to me, the extension I have less insight into though

[martinvuyk]

@JoeLoser now that I'm going through the stdlib searching for places where we should use UInt instead of Int I came across this:

        # TODO: Simplify this with a UInt type.
        debug_assert(
            -self._len <= Int(idx) < self._len, "index must be within bounds"
        )
        # TODO(MSTDL-1086): optimize away SIMD/UInt normalization check
        var offset = Int(idx)
        if offset < 0:
            offset += len(self)
        return self._data[offset]

I already changed it to:

        var offset = UInt(idx.__index__())
        # TODO(MSTDL-1086): optimize away SIMD normalization check
        if not _type_is_eq[I, UInt]():
            if Int(idx) < 0:
                offset = len(self) + Int(idx)
        debug_assert(offset < self._len, "index must be within bounds")
        return self._data[offset]

MSTDL-1086 would be solved by doing:

        var offset = UInt(idx.__index__())
        alias dt = DType.get_dtype[I]()

        @parameter
        if not _type_is_eq[I, UInt]() and not dt.is_unsigned():
            if Int(idx) < 0:
                offset = len(self) + Int(idx)
        debug_assert(offset < self._len, "index must be within bounds")
        return self._data[offset]

[JoeLoser]

!sync

[JoeLoser]

!sync

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[modularbot]

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