Instead of rescanning debug_info every time we want to read a function
(either to find inlined calls or its variables) cache the tree of
dwarf.Entry that we would generate and use that.

Benchmark before:

BenchmarkConditionalBreakpoints-4   	       1	5164689165 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	4817425836 ns/op

Updates #1549