memoise Text calls

[plastikfan]

See:

• memoization in golang

Not all message entries are realistically memoizable. Any messages with dynamic content should not be memoized. Each xxxTemplData entry could have a flag set on them to indicate if it can be memoized. Or we could introduce a new struct that can be composed in which indicates it is memoizable. When Text is called, it will check if it is memoizable and take appropriate action.

tcache?? (Similar to the Linux dcache used to cache inodes to speed up file system performance)

Sync.Map