Reduce code size of {f32,f64}::is_finite

[0f-0b]

This change eliminates the two constants (the abs mask and infinity), reducing code size at hopefully no performance cost. It works because x - x is always +0 when x is finite, and is NaN if and only if x is infinite or NaN.

Generated aarch64 assembly for f32 compared to the current implementation:

is_finite:
    fsub  s0, s0, s0
    fcmp  s0, s0
    cset  w0, vc
    ret

is_finite_std:
    fmov  w9, s0
    mov   w8, #0x7f800000
    and   w9, w9, #0x7fffffff
    cmp   w9, w8
    cset  w0, lt
    ret

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