* Rename `ILBuilder` to `FunctionBuilderContext` and update corresponding
code
* Refactor usages of ILBuilder to become FunctionBuilderContext,
update variable names to reflect this change
* Reformat to ensure that lines stay under 100 char limit
* Apply corrections from `rustfmt` to pass tests
* Rename variables to be more consistent with refactor of ILBuilder
This allows the assertions to be disabled in release builds, so that
the code is faster and smaller, at the expense of not performing the
checks. Assertions can be re-enabled in release builds with the
debug-assertions flag in Cargo.toml, as the top-level Cargo.toml
file does.
Previously, cretonne-wasm used its own Local struct for identifying
local variables. However, now that cretonne-frontend provides a
Variable struct, just use that instead.
Frontends can still use their own types with `ILBuilder` and
`FunctionBuilder`. This just provides a basic `Variable` struct
for frontends that want it.
