The `units.parse` builtin should not round to the value to the nearest integer

`units.parse("1.0005k")` outputs `1000` instead of `1000.5`. This is inconsistent with the Go implementation: https://play.openpolicyagent.org/p/TBQcYuuaQr

More context here: https://github.com/matrix-org/rust-opa-wasm/pull/30#pullrequestreview-1058803684

Start by locating the Rust implementation of the `units.parse` builtin and compare its behavior with the linked Go implementation. Reproduce `units.parse("1.0005k")` and verify that the result retains the fractional value `1000.5` rather than rounding to `1000`.

Written by the indexing model from the issue text.