Compiler: simplify word size testing

[chqrlie]

• add native_kind in ast_builder.Builder
• group allocation stuff in TailAllocator as a member of ast_context.Context
• add wordsize in ast_context.Context
• move compiler sub-object creation after target analysis
• use gen.targetInfo for word size in C Generator
• add align member in Type.typebits, use for builtins, pointers and functions
• initialize dummy slot 0 in pointer pool
• move getWordSize() and getNativeKind() from global to ast_context
• simplify size_analyser.sizeOfType: no reference to global needed
• conversion_checker gets native_kind from builder/context
• simplify getUnaryOpWidth, no global reference anymore

[bvdberg]

What's the purpose of splitting Context into a TailAllocator and a Context?

[chqrlie]

The TailAllocator belongs in ast_utils and should only deal with allocation

The ast_context.Context should hold the core ast data, such as the builtins, the string and pointer pool, and some ancillary data such as the wordsize. It should be moved to the ast module as ast.AstContext and passed to the different modules.

This PR performs an initial step in this direction but it might be better to make a more radical move, for which I have a larger patch in progress.