Extend Printer::buf on demand.

[nnethercote]

So that 55 entries (at 48 bytes each) don't need to be eagerly
initialized on creation.

This speeds up numerous rust-perf benchmark runs, by up to 3%.

crates.io-check
        avg: -2.4%      min: -3.7%      max: -1.1%
encoding-check
        avg: -2.1%      min: -2.9%      max: -1.2%
crates.io-opt
        avg: -1.3%      min: -2.7%      max: -0.1%
crates.io
        avg: -1.4%      min: -2.7%      max: -0.3%
encoding-opt
        avg: -1.1%      min: -2.5%      max: 0.1%
encoding
        avg: -1.3%      min: -2.4%      max: -0.3%
hyper-check
        avg: -1.7%      min: -2.3%      max: -0.9%
regex-check
        avg: -1.5%      min: -1.9%      max: -0.7%
piston-image-check
        avg: -0.9%      min: -1.8%      max: -0.5%
hyper
        avg: -1.0%      min: -1.7%      max: -0.3%
hyper-opt
        avg: -0.9%      min: -1.7%      max: -0.1%
syn-check
        avg: -1.0%      min: -1.5%      max: -0.6%
clap-rs
        avg: -0.3%      min: -1.5%      max: 0.2%
regex-opt
        avg: -0.6%      min: -1.5%      max: -0.0%
regression-31157-check 
        avg: -1.1%      min: -1.4%      max: -0.7%
regex
        avg: -0.7%      min: -1.3%      max: -0.1% 
clap-rs-check
        avg: -0.5%      min: -1.2%      max: 0.1%
syn-opt
        avg: -0.5%      min: -1.1%      max: -0.1%
syn
        avg: -0.5%      min: -1.1%      max: -0.2%
serde-opt
        avg: -0.3%      min: -1.1%      max: 0.1%
piston-image-opt
        avg: -0.4%      min: -1.1%      max: -0.0%
piston-image
        avg: -0.4%      min: -1.0%      max: -0.0%

[rust-highfive]

r? @michaelwoerister

(rust_highfive has picked a reviewer for you, use r? to override)

[michaelwoerister]

Oh yeah, I remember seeing this when profiling compilations of Stylo. Thanks, @nnethercote!

@bors r+

[bors]

📌 Commit 989815d has been approved by michaelwoerister

[bors]

⌛ Testing commit 989815d with merge 6a87289...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: michaelwoerister
Pushing 6a87289 to master...