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Use two internal representations for Float values (similar to what is done
for Int values). Transparently switch to a big.Float representation when
big.Rat values become unwieldy. This is almost never needed for real-world
programs but it is trivial to create test cases that cannot be handled with
rational arithmetic alone.

As a consequence, the go/constant API semantics changes slightly: Until now,
a value could always be represented in its "smallest" form (e.g., float values
that happened to be integers would be represented as integers). Now, constant
Kind depends on how the value was created, rather than its actual value. (The
reason why we cannot automatically "normalize" values to their smallest form
anymore is because floating-point numbers are not exact in general; and thus
normalization is often not possible in the first place, or would throw away
precision when it is not desired.) This has repercussions as to ho...