Constant time struct field access

[zslayton]

Issue #, if available:
#672

Description of changes:

This PR modifies the TemplateCompiler to maintain a field_name => field_value_addresses index for each template struct that it encounters. This allows LazyStruct::find(name: &str) to perform a single HashMap lookup any time its backing data is a template. If its backing data is a value literal, it will still need to perform a linear scan.

Commit guide:

• f986c14 contains the meat of the change
• 576fb0b removes a now-superflous context parameter from a few MacroEvaluator methods; MacroEvaluator stores its own context that can be used instead as needed.
• 91f2d25 Incorporates clippy suggestions

Here are some timings from of the included benchmark from my 2019 Intel MBP:

Data Version	Scan all fields	Read single field	Read all fields
Ion 1.0	118ms	123ms	138ms
Ion 1.1	40ms	42ms	67ms

The same benchmarks, this time running on an m5.4xlarge:

Data Version	Scan all fields	Read single field	Read all fields
Ion 1.0	133ms	139ms	157ms
Ion 1.1	46ms	48ms	75ms

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[popematt]

Looks great! I am curious though, do you have any before and after benchmarks for this change (as opposed to Ion 1.0 vs Ion 1.1)? I'm curious to know how much of the difference between 1.0 and 1.1 is attributable to this change.

[zslayton]

Looks great! I am curious though, do you have any before and after benchmarks for this change (as opposed to Ion 1.0 vs Ion 1.1)? I'm curious to know how much of the difference between 1.0 and 1.1 is attributable to this change.

On my laptop:

Change	Scan all fields	Read single field	Read all fields
Before PR	40ms	50ms	67ms
After PR	40ms	42ms	67ms