Speed up qp problem encoding

[randomir]

ASV benchmarks on my machine:

• active_qubits(): 34 ms -> 22 ms
• active_qubits() on BQM: 156 ms -> 121ms
• encode_problem_as_qp(): 103 ms -> 75 ms
• encode_problem_as_qp() on BQM: 650 ms -> 560 ms and then -> 400 ms when converted to dict upfront

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.86%. Comparing base (5597ff0) to head (91b662e).
Report is 55 commits behind head on master.

Files with missing lines	Patch %	Lines
dwave/cloud/coders.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #661      +/-   ##
==========================================
- Coverage   88.91%   88.86%   -0.06%     
==========================================
  Files          42       42              
  Lines        4810     4815       +5     
==========================================
+ Hits         4277     4279       +2     
- Misses        533      536       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.