make Compiler thread-safe by moving CodeBuilder to instance scope

[yaodingyd]

I'm using compilers in a supposedly thread-safe way:

def _get_compiler():
    compiler = getattr(_thread_local, "compiler", None)
    if compiler is None:
        compiler = Compiler()
        _thread_local.compiler = compiler
    return compiler

# then in subsequent code
_get_compiler().compile(my_text)

but running above code in a threadpool still running into issue

....
pybars/_compiler.py", line 629, in find_lookup
    self._result.grow([
    ^^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute 'grow'

I think this is because each Compiler instance still reference to the same class field of _builder and _compiler, so their state get mutated cross thread

This change move these two fields to be instance property so when creating Compiler instance for each thread, each instance get isolated _build and _compiler

I don't have context on why we are keeping them this way so please correct me, appreciate more historical insights here