Reuse buffer with sync.Pool

[kramaranya]

While investigating a task #334 to consider sync.Pool in our code, I discovered that currently, each invocation of templateWithValue creates a new bytes.Buffer and discards it. By switching to a sync.Pool to reuse and reset these buffers, we can cut down on allocations and reduce overhead.

Here are the benchmark results I gathered using benchstat to compare before_change.txt (no sync.Pool) versus after_change.txt (using sync.Pool):

name                old time/op    new time/op    delta
TemplateWithValue-12    171.5ns ± 4%   145.4ns ± 0%  -15.19% (p=0.000 n=10)

name                old alloc/op   new alloc/op   delta
TemplateWithValue-12    192B ± 0%     80B ± 0%    -58.33% (p=0.000 n=10)

name                old allocs/op  new allocs/op  delta
TemplateWithValue-12      6.0 ± 0%     4.0 ± 0%   -33.33% (p=0.000 n=10)

Reusing the buffer with sync.Pool is better because it lowers overhead from creating new buffers on each call, reduces memory usage and helps the garbage collector by cutting down on short-lived objects:

• Time per operation improved by about 15%.
• Memory usage dropped by more than 50%.
• Number of allocations reduced from 6 to 4.

[ibihim]

Maybe I am a burned child as I worked with JavaScript in the crazy days, but usually you avoid having a "global" variable.
As it is local to the package, it might be okaish, if we would rename it to something like "templateBufferPool".

A better solution would be to initiate this in the constructor and pass it down from the struct into the templateWithValue function. According to the docs, we must not copy it, so we would need to use a pointer.

[kramaranya]

Thanks, I've addressed your suggestion