Allows use of slightly optimized variants that check if they have to use
the heavier vte parser. Tries to avoid unnnecessary allocations. Initial
performance characteristics proven out in #4378.
Also reduces boilerplate with right-ward drift.
Same rationale as in #4378
Also accelerate `grid`
before:
```
Command being timed: "./eager_nu -c for i in 0..100000 { echo whatever } | grid"
User time (seconds): 0.21
System time (seconds): 0.05
Percent of CPU this job got: 36%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.71
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 48112
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 10580
Voluntary context switches: 266
Involuntary context switches: 2595
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
```
after:
```
Command being timed: "./lazy_nu -c for i in 0..100000 { echo whatever } | grid"
User time (seconds): 0.14
System time (seconds): 0.05
Percent of CPU this job got: 33%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.60
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 48272
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 1
Minor (reclaiming a frame) page faults: 10582
Voluntary context switches: 286
Involuntary context switches: 831
Swaps: 0
File system inputs: 56
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
```
