[19_5] improve performance of resize and provide reserve

[jingkaimori]

Works

• introduce reserve method;
  • works like std::basic_string::reserve
• holds size of array in a_N field;
  • works like std::basic_string::capacity
• introduce expand_or_shrink_by method to accelerate append of string.
  • expand_or_shrink_by avoid re-allocation whenever possible, so if a large string is shrink by expand_or_shrink_by, memory is not released.
  • resize works like std::basic_string::resize, sometimes resize can be used as std::basic_string::shrink_to_fit.
• add corresponding tests

Example

use reserve() along with append operator:

string r;
r.reserve(100);
for(int i=0; i<100; i++){
    r << (i%26) + 'a';
}

Performance

Before

ns/op	op/s	err%	total	benchmark
79.08	12,645,769.22	0.8%	0.95	construct string
42.26	23,660,680.56	2.5%	0.71	append string

After

ns/op	op/s	err%	total	benchmark
75.58	13,231,790.34	0.9%	0.90	construct string
20.95	47,728,672.38	2.5%	0.47	append string

[da-liii]

Could you explain why we need to add two extra interfaces? Are there similar designs in other C++ string libraries?