refactor: use CodeBuilder

[igamigo]

I think #187 got closed when the base branch got merged.

Official guidance should be to utilize the CodeBuilder, which is also provided by Client. Recently mixing source managers in assemblers started resulting in internal panics (0xMiden/miden-vm#2778), so we need to make guidance as consistent as possible. This also simplifies code a bunch. cc @Keinberger @BrianSeong99

Closes #188.

[bitwalker]

Just a quick aside - but I'd recommend switching over to using projects (i.e. defined by a miden-project.toml file) and the project assembler in our tutorials, rather than manually assembling things. Not only is the API surface way simpler that way, but you automatically get a Miden package as output.

I'm hoping to expand miden build by the next release so that it can assemble both Rust and MASM projects directly, so it should basically never be necessary to construct an Assembler directly as an end user to get a package.