Extend Printer::buf on demand.
Extend Printer::buf on demand.

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%.

GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust