Implement compound expressions and arithmetic operators

[jg-rp]

This PR implements compound expressions and arithmetic operators.

Compound expression contain logical operators (and and or) and/or relational operators (==, >, etc.). They can appear in most places where a primitive expressions would previously have been used, including in output statements, in assign tags and as arguments to filters.

See Shopify/liquid#1922 and jg-rp/liquid#175.

For example, we can now write this:

{{ user.name or "guest" }}

Whereas before we'd need an {% if %} tag.

{% if user.name -%}
  {{ user.name }}
{%- else -%}
   guest
{%- endif %}

Arithmetic infix operators +, -, *, /, % and **, and prefix operators + and -, are disabled by default. Enable them by setting arithmetic_operators to True in an Environment subclass.

from liquid2 import Environment

class MyEnvironment(Environment):
    arithmetic_operators = True

env = MyEnvironment()
template = env.from_string("{{ a + b * c }}")
print(template.render(a=1, b=2, c=3))  # 7

Without arithmetic operators, the equivalent expression would be {{ b | times: c | plus: a }}.