Add polynomial string interpolation support.

This change adds support for the poly"..." interpolator. For example:

    import spire.implicits._

    val i = 99.3
    val j = r"3/17"
    val k = BigInt(12345)

    poly"${i}x^2 + ${j}x + $k"

This will produce (993/10x^2 + 3/17x + 12345). Any value whose toString
produces a valid rational number will work, even things like:

    val s = "5"
    poly"${s}x + 1"

For now, this is all happening at runtime, although it would be
possible to write a macro that evaluates these at compile-time, when
there is no interpolation, e.g.:

    poly"x^2 + 3x - 6"

Also, this commit adds some cute new factory methods:

    Polynomial.constant(3.0)
    Polynomial.linear(r"44/7")
    Polynomial.quadratic(4)
    Polynomial.cubic(BigDecimal(16.23))