This reworks some of WebAPIResource to reduce unnecessary busywork and

reorganize some blocks of code to help with maintenance, performance,

and memory.
Initial state setup for resources no longer allocates more memory for

allowed_mimetypes when there's no vendor required for resources

(though a vendor is recommended).
Dynamically-created state caches and key lookups are now accessed via a

try/except rather than a series of hasattr or in calls, which is

faster overall and especially in the common case. Some caches are now

built with reduced loops or overhead.
Object serialization now computes all request-specific code (cache

setup, expanded resources) up-front in one if request:, in order to

make it clear what's state setup and what's payload building.
Some unnecessary isinstance checks, if conditions, attribute

lookups, and list allocations have been removed.
Some string building has been replaced with f-strings.
Small optimizations that add up, saving a few seconds in my typical unit

test runs for Review Board's API test suite.

Djblets and Review Board test suites pass.