Clarify naming for builds and evaluations

[jeremyfowers]

Problem statement

the API is named "add_build_stat()" - that should be renamed to something more general

I was thinking of evaluation as a superset of build+benchmark

Proposal

• in the stats.yaml: builds -> evaluations
• add_build_stat() -> save_model_evaluation_stat()
• stat_id -> evaluation_id
• save_stat() -> save_model_stat()

cc @danielholanda