language: Added approximate number literals (#185)
This turned out to be a much more involved change that I expected. It
started by adding syntax support for scientific notation (i.e. 32e5) but
ended up overhauling how numbers work in vsql, and solving some major
current and future problems.
All literals in scientific notation are considered `DOUBLE PRECISION`,
that part is pretty straightforward. Furthermore, all approximate
numbers are always formatted in scientific notation, even if smaller
numbers are given a "e0" suffix.
Since this means that exact and approximate numbers now have a different
form we can take a lot of hacky and unreliable guess work out of
literals and implicit casting.
`REAL` and `DOUBLE PRECISION` no longer need to be treated as supertypes
for all other exact types. Which in retrospect makes no sense. The docs
make much more sense on how numbers work and this makes the larger
change of DECIMAL and NUMERIC types coming later much easier to explain
and implement.
