Improve speed & memory use for Diagnose() and DiagnoseFirst()

[benluddy]

Description

This PR improves efficiency of functions in diagnose.go used by Diagnose() and DiagnoseFirst(), especially related to use of encoding/hex, encoding/base32, and encoding/base64.

The "Encoder" types for the various byte string diagnostic encodings (encoding/hex, encoding/base32,
and encoding/base64) include sizable internal buffers (1KiB at time of writing), and a new encoder
is created to produce the diagnostic encoding of every byte string in the input. This results in a
lot of extra allocations, especially for inputs containing many small byte strings.

                                        │  before.txt  │              after.txt              │
                                        │    sec/op    │   sec/op     vs base                │
Diagnose/byte_string_base16_encoding      315.90n ± 0%   95.03n ± 1%  -69.92% (p=0.000 n=10)
Diagnose/byte_string_base32_encoding      325.55n ± 0%   99.27n ± 1%  -69.51% (p=0.000 n=10)
Diagnose/byte_string_base32hex_encoding    325.4n ± 0%   100.2n ± 1%  -69.21% (p=0.000 n=10)
Diagnose/byte_string_base64url_encoding    336.5n ± 0%   100.7n ± 2%  -70.07% (p=0.000 n=10)
geomean                                    325.8n        98.78n       -69.68%

                                        │  before.txt  │             after.txt              │
                                        │     B/op     │    B/op     vs base                │
Diagnose/byte_string_base16_encoding      1344.00 ± 0%   80.00 ± 0%  -94.05% (p=0.000 n=10)
Diagnose/byte_string_base32_encoding      1344.00 ± 0%   80.00 ± 0%  -94.05% (p=0.000 n=10)
Diagnose/byte_string_base32hex_encoding   1344.00 ± 0%   80.00 ± 0%  -94.05% (p=0.000 n=10)
Diagnose/byte_string_base64url_encoding   1344.00 ± 0%   80.00 ± 0%  -94.05% (p=0.000 n=10)
geomean                                   1.313Ki        80.00       -94.05%

                                        │ before.txt │             after.txt              │
                                        │ allocs/op  │ allocs/op   vs base                │
Diagnose/byte_string_base16_encoding      5.000 ± 0%   2.000 ± 0%  -60.00% (p=0.000 n=10)
Diagnose/byte_string_base32_encoding      5.000 ± 0%   2.000 ± 0%  -60.00% (p=0.000 n=10)
Diagnose/byte_string_base32hex_encoding   5.000 ± 0%   2.000 ± 0%  -60.00% (p=0.000 n=10)
Diagnose/byte_string_base64url_encoding   5.000 ± 0%   2.000 ± 0%  -60.00% (p=0.000 n=10)
geomean                                   5.000        2.000       -60.00%

Producing the \uxxxx escape sequences also made a few more allocations than necessary.

                                          │ before.txt  │              after.txt              │
                                          │   sec/op    │   sec/op     vs base                │
Diagnose/escaped_character_in_text_string   180.3n ± 0%   109.2n ± 2%  -39.41% (p=0.000 n=10)

                                          │ before.txt  │             after.txt              │
                                          │    B/op     │    B/op     vs base                │
Diagnose/escaped_character_in_text_string   192.00 ± 0%   72.00 ± 0%  -62.50% (p=0.000 n=10)

                                          │ before.txt │             after.txt              │
                                          │ allocs/op  │ allocs/op   vs base                │
Diagnose/escaped_character_in_text_string   5.000 ± 0%   2.000 ± 0%  -60.00% (p=0.000 n=10)

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue #___
• Closes or Updates Issue #___

Checklist (for code PR only, ignore for docs PR)

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[fxamacker]

@benluddy LGTM! Thanks for the optimization and benchmark results is very impressive!