[Bundlex] Refactor Unix toolchain

[mat-hek]

The internal abstractions in Bundlex are quite complex and out of date:

• All abstractions over various toolchains except Windows call the common Unix module in the end
• Some callbacks are never used

This increases the entry level and slows down the development. The idea to fix it is the following:

• Remove and simplify abstractions (Bundlex.Platform in particular)
• Create two main, separate toolchains: Windows and Unix
• Possibly create a single compilation state (either global or per toolchain) that would be passed around, so that all the needed data was easily accessible for different modules