Consolidate contexts

[Mercerenies]

We have too many "context" classes.

• CommandContext
• FunctionContext
• SimplifierContext
• ValidationContext
• QueryContext

Consolidate and clarify what each one means.

[Mercerenies]

Current shapes (as of 9de2c93) of the relevant structs:

pub struct CommandContext<'a, 'b> {
  pub opts: CommandOptions,
  pub simplifier: Box<dyn Simplifier + 'a>,
  pub units_parser: &'b dyn UnitParser<Number>,
}

pub struct FunctionContext<'a, 'b, 'c> {
  pub errors: &'a mut ErrorList<SimplifierError>,
  pub simplifier: &'b dyn Simplifier,
  pub function_table: &'c FunctionTable,
  _private: (),
}

pub struct ValidationContext<'a, 'b> {
  pub units_parser: &'a dyn UnitParser<Number>,
  pub language_mode: &'b dyn LanguageMode,
}

pub struct QueryContext<'a> {
  pub units_parser: &'a dyn UnitParser<Number>,
}

pub struct SimplifierContext<'a, 'b> {
  pub base_simplifier: &'a dyn Simplifier,
  pub errors: &'b mut ErrorList<SimplifierError>,
}

[Mercerenies]

And here's where we construct each of them:

CommandContext:

• run_math_command
• impl Default for CommandContext

FunctionContext:

• call and call_for_graphics

ValidationContext:

• validate_value
• Unit Tests

QueryContext:

• query_stack
• Unit Tests

SimplifierContext:

• CommandContext::simplify_expr
• ExprFunction and ExprFunction2 in crate::expr::algebra