Optimize frequently inlined FBString methods

Optimize frequently inlined FBString methods

Summary:
Almost every method of `fbstring` needs to perform category
dispatching. The category constants are `size_t`, which become 8-byte
immediate values in the dispatching code, so even a simple `if
(category() == Category::isSmall)` is quite large. When inlined
hundreds of thousands of time, it adds up.

This diff redefines the category type to be 1 byte (without changing
the ABI). It also optimizes `size()` and `c_str()` and makes them
branch-free, which probably is not going to have any perf impact but
it saves a few bytes.

Generated code for some small functions:

- `reset()`
Before:
```
48 ba 00 00 00 00 00    movabs $0x1700000000000000,%rdx
00 00 17
48 89 f8                mov    %rdi,%rax
c6 07 00                movb   $0x0,(%rdi)
48 89 57 10             mov    %rdx,0x10(%rdi)
```

20 bytes

After:
```
48 89 f8                mov    %rdi,%rax
c6 47 17 17             movb   $0x17,0x17(%rdi)
c6 07 00                movb   $0x0,(%rdi)
```

10 bytes

- `c_str()`
Before:
```
48 b8 00 00 00 00 00    movabs $0xc000000000000000,%rax
00 00 c0
48 85 47 10             test   %rax,0x10(%rdi)
74 08                   je     401fd8
48 8b 07                mov    (%rdi),%rax
c3                      retq
0f 1f 40 00             nopl   0x0(%rax)
48 89 f8                mov    %rdi,%rax
```

26 bytes (without the `retq`)

After:
```
f6 47 17 c0             testb  $0xc0,0x17(%rdi)
48 89 f8                mov    %rdi,%rax
48 0f 45 07             cmovne (%rdi),%rax
```

11 bytes

- `size()`
Before:
```
48 b8 00 00 00 00 00    movabs $0xc000000000000000,%rax
00 00 c0
48 85 47 10             test   %rax,0x10(%rdi)
74 08                   je     401fa8
48 8b 47 08             mov    0x8(%rdi),%rax
c3                      retq
0f 1f 00                nopl   (%rax)
48 0f be 57 17          movsbq 0x17(%rdi),%rdx
b8 17 00 00 00          mov    $0x17,%eax
48 29 d0                sub    %rdx,%rax
```

36 bytes (without the `retq`)

After:
```
0f b6 57 17             movzbl 0x17(%rdi),%edx
b8 17 00 00 00          mov    $0x17,%eax
48 29 d0                sub    %rdx,%rax
48 0f 48 47 08          cmovs  0x8(%rdi),%rax
```

17 bytes

Reviewed By: philippv

Differential Revision: D3957276

fbshipit-source-id: ef40d82bbbb0456b1044421cd02133c268abe39b