buffer: optimize readDouble and readFloat methods

Compute the floating point number in JavaScript to avoid having to call
out to the C++ runtime.  The improvements are not insubstantial:

                                                             improvement confidence      p.value
    value="big" endian="BE" type="Double" noAssert="false"      292.86 %        *** 1.688367e-08
    value="big" endian="BE" type="Double" noAssert="true"       353.19 %        *** 6.079414e-10
    value="big" endian="BE" type="Float" noAssert="false"       406.21 %        *** 1.730122e-07
    value="big" endian="BE" type="Float" noAssert="true"        450.81 %        *** 6.909242e-07
    value="big" endian="LE" type="Double" noAssert="false"      268.39 %        *** 8.625486e-09
    value="big" endian="LE" type="Double" noAssert="true"       310.66 %        *** 2.798332e-15
    value="big" endian="LE" type="Float" noAssert="false"       382.99 %        *** 3.412057e-07
    value="big" endian="LE" type="Float" noAssert="true"...